Showing 1–50 of 54 results for author: Linke, H

Combined Light Excitation and Scanning Gate Microscopy on Heterostructure Nanowire Photovoltaic Devices

Authors: Yen-Po Liu, Jonatan Fast, Yang Chen, Ren Zhe, Adam Burke, Rainer Timm, Heiner Linke, Anders Mikkelsen

Abstract: Nanoscale optoelectronic components achieve functionality via spatial variation in electronic structure induced by composition, defects, and dopants. To dynamically change the local band alignment and influence defect states, a scanning gate electrode is highly useful. However, this technique is rarely combined with photoexcitation by a controlled external light source. We explore a setup that com… ▽ More Nanoscale optoelectronic components achieve functionality via spatial variation in electronic structure induced by composition, defects, and dopants. To dynamically change the local band alignment and influence defect states, a scanning gate electrode is highly useful. However, this technique is rarely combined with photoexcitation by a controlled external light source. We explore a setup that combines several types of light excitation with high resolution scanning gate and atomic force microscopy (SGM/AFM). We apply the technique to InAs nanowires with an atomic scale defined InP segment, that have attracted considerable attention for studies of hot carrier devices. Using AFM we image the topography of the nanowire device. SGM measurements without light excitation show how current profiles can be influenced by local gating near the InP segment. Modelling of the tip and nanowire can well predict the results based on the axial band structure variation and an asymmetric tip. SGM studies including light excitation are then performed using both a white light LED and laser diodes at 515 and 780nm. Both negative and positive photoconductance can be observed and the combined effect of light excitation and local gating is observed. SGM can then be used to discriminate between effects related to the wire axial compositional structure and surface states. The setup explored in the current work has significant advantages to study optoelectronics at realistic conditions and with rapid turnover. △ Less

Submitted 19 February, 2025; originally announced February 2025.

Error-rate reduction in network-based biocomputation

Authors: Pradheebha Surendiran, Marko Ušaj, Till Korten, Alf Månsson, Heiner Linke

Abstract: Network-based biocomputation (NBC) is an alternative parallel computing paradigm that encodes combinatorial problems into a nanofabricated device's graphical network of channels, enabling cytoskeletal filaments propelled by molecular motors to explore the problems' solution space. NBC promises to require significantly less energy than traditional computers due to the high energy efficiency of mole… ▽ More Network-based biocomputation (NBC) is an alternative parallel computing paradigm that encodes combinatorial problems into a nanofabricated device's graphical network of channels, enabling cytoskeletal filaments propelled by molecular motors to explore the problems' solution space. NBC promises to require significantly less energy than traditional computers due to the high energy efficiency of molecular motors. However, error rates associated with the pass junction crossing, the primary path-regulating geometry, pose a bottleneck for scaling up this technology. Here, we optimize the geometry of the pass junction for low error rates for the actin-myosin system. To do so, we evaluate various pass junction designs that differ in features, such as the nanochannel width, junction crossing area, and angles of a funnel-shaped output part of the junction. Error rates were measured experimentally by using gliding motility assay and as well as by simulation methods. The final optimized design displayed a decreased error rate of under 1 percent compared to the previous 2-4 percent. We anticipate this improvement will enable scaling up NBC networks from tens to hundreds of pass junctions. However, the results of 2D junction optimizations also suggest that further drastic reduction of error rates in two-dimensional pass junctions is unlikely, necessitating three-dimensional junctions, such as bridges or tunnels, for complete error rate mitigation. Furthermore, the simulation results demonstrate that including a layer of myosin motor on the channel provides a better fit between simulation and experimental results. △ Less

Submitted 1 July, 2024; originally announced July 2024.

Comments: 22 pages plus supplementary information, 6 figures

Geometric symmetry breaking and nonlinearity can increase thermoelectric power

Authors: Jonatan Fast, Hanna Lundström, Sven Dorsch, Lars Samuelson, Adam Burke, Peter Samuelsson, Heiner Linke

Abstract: Direct thermal-to-electric energy converters typically operate in the linear regime, where the ratio of actual maximum power relative to the ideal maximum power, the so-called fill factor (FF), is 0.25. By increasing the FF one can potentially increase maximum power by up to four times, but this is only possible in the nonlinear regime of transport and has previously rarely been considered. Here w… ▽ More Direct thermal-to-electric energy converters typically operate in the linear regime, where the ratio of actual maximum power relative to the ideal maximum power, the so-called fill factor (FF), is 0.25. By increasing the FF one can potentially increase maximum power by up to four times, but this is only possible in the nonlinear regime of transport and has previously rarely been considered. Here we show, based on fundamental symmetry considerations, that the leading order non-linear terms that can increase the FF require devices with broken spatial symmetry. To experimentally demonstrate such a system, we study nonlinear, thermoelectric transport across an asymmetric energy barrier epitaxially defined in a single semiconductor nanowire. We find in both experiment and theory that we can increase the FF above the linear-response limit of 0.25, accompanied by a drastic increase in short circuit current, open-circuit voltage and maximum power. Our results show that geometric symmetry breaking combined with the design of nonlinear behaviour represent a design strategy for increasing the performance of thermal-to-electric energy converters such as in hot-carrier photovoltaics, thermophotovoltaics or in anisotropic thermoelectric materials. △ Less

Submitted 4 April, 2023; originally announced April 2023.

Comments: 11 pages, 3 figures. Submitted to Nature Physics

Dissipation reduction and information-to-measurement conversion in DNA pulling experiments with feedback protocols

Authors: Marc Rico-Pasto, Regina K. Schmitt, Marco Ribezzi-Crivellari, Juan M. R. Parrondo, Heiner Linke, Jonas Johansson, Felix Ritort

Abstract: Information-to-energy conversion with feedback measurement stands as one of the most intriguing aspects of the thermodynamics of information in the nanoscale. To date, experiments have focused on feedback protocols for work extraction. Here we address the novel case of dissipation reduction in non-equilibrium systems with feedback. We perform pulling experiments on DNA hairpins with optical tweeze… ▽ More Information-to-energy conversion with feedback measurement stands as one of the most intriguing aspects of the thermodynamics of information in the nanoscale. To date, experiments have focused on feedback protocols for work extraction. Here we address the novel case of dissipation reduction in non-equilibrium systems with feedback. We perform pulling experiments on DNA hairpins with optical tweezers, with a general feedback protocol based on multiple measurements that includes either discrete-time or continuous-time feedback. While feedback can reduce dissipation, it remains unanswered whether it also improves free energy determination (information-to-measurement conversion). We define thermodynamic information Υ as the natural logarithm of the feedback efficacy, a quantitative measure of the efficiency of information-to-energy and information-to-measurement conversion in feedback protocols. We find that discrete and continuous-time feedback reduces dissipation by roughly kBTΥ without improvement in free energy determination. Remarkably, a feedback strategy (defined as a correlated sequence of feedback protocols) further reduces dissipation, enhancing information-to-measurement efficiency. Our study underlines the role of temporal correlations to develop feedback strategies for efficient information-to-energy conversion in small systems. △ Less

Submitted 14 October, 2021; originally announced October 2021.

Comments: main text: 20 pages (including appendixes), 11 figures and 2 tables. supplementary material: 7 pages and 3 figures

Journal ref: Phys. Rev. X 11(3), 031052 (2021)

Information-to-work conversion in single molecule experiments: from discrete to continuous feedback

Authors: Regina K. Schmitt, Patrick P. Potts, Heiner Linke, Marc Rico-Pasto, Jonas Johansson, Felix Ritort, Peter Samuelsson

Abstract: We theoretically investigate the extractable work in single molecule unfolding-folding experiments with applied feedback. Using a simple two-state model, we obtain a description of the full work distribution, from discrete to continuous feedback. The effect of the feedback is captured by a detailed fluctuation theorem, accounting for the information aquired. We find analytical expressions for the… ▽ More We theoretically investigate the extractable work in single molecule unfolding-folding experiments with applied feedback. Using a simple two-state model, we obtain a description of the full work distribution, from discrete to continuous feedback. The effect of the feedback is captured by a detailed fluctuation theorem, accounting for the information aquired. We find analytical expressions for the average work extraction as well as an experimentally measurable bound thereof, which becomes tight in the continuous feedback limit. We further determine the parameters for maximal power, or rate of work extraction. While our two-state model only depends on a single, effective transition rate, we find quantitative agreement with Monte Carlo simulations of DNA hairpin unfolding-folding dynamics. △ Less

Submitted 12 October, 2021; originally announced October 2021.

Comments: 5 pages, 4 figures, 5 pages of supplementary information

Journal ref: Phys. Rev. E 107, L052104 (2023)

Hot-carrier optoelectronic devices based on semiconductor nanowires

Authors: Jonatan Fast, Urs Aeberhard, Stephen P. Bremner, Heiner Linke

Abstract: In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs are generated as so called hot carriers with an excess energy that is typically lost as heat. The long standing aim to harvest this excess energy to enhance device performance has proven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on inc… ▽ More In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs are generated as so called hot carriers with an excess energy that is typically lost as heat. The long standing aim to harvest this excess energy to enhance device performance has proven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on increasing the hot carrier relaxation time, and on tailoring heterostructures that allow for hot-carrier extraction on short time- and length-scales. Recently, semiconductor nanowires have emerged as a promising system to achieve these aims, because they offer unique opportunities for heterostructure engineering as well as for potentially modified phononic properties that can lead to increased relaxation times. In this review we assess the current state of theory and experiments relating to hot-carrier dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a foundation, we begin with a brief overview of the fundamental processes involved in hot-carrier relaxation, and how these can be tailored and characterized in nanowires. We then analyze the advantages offered by nanowires as a system for hot-carrier devices and review the status of proof-of-principle experiments related to hot-carrier photovoltaics. To help interpret existing experiments on photocurrent extraction in nanowires we provide modelling based on non-equilibrium Green's functions. Finally, we identify open research questions that need to be answered in order to fully evaluate the potential nanowires offer towards achieving more efficient, hot-carrier based, optoelectronic devices. △ Less

Submitted 19 April, 2021; originally announced April 2021.

Comments: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 8, 021309 (2021), and may be found at https://doi.org/10.1063/5.0038263

Journal ref: Appl. Phys. Rev. 8, 021309 (2021)

Prospects for single-molecule electrostatic detection in molecular motor gliding motility assays

Authors: M. Sanchez Miranda, R. Lyttleton, P. H. Siu, S. Diez, H. Linke, A. P. Micolich

Abstract: Molecular motor gliding motility assays based on myosin/actin or kinesin/microtubules are of interest for nanotechnology applications ranging from cargo-trafficking in lab-on-a-chip devices to novel biocomputation strategies. Prototype systems are typically monitored by expensive and bulky fluorescence microscopy systems and the development of integrated, direct electric detection of single filame… ▽ More Molecular motor gliding motility assays based on myosin/actin or kinesin/microtubules are of interest for nanotechnology applications ranging from cargo-trafficking in lab-on-a-chip devices to novel biocomputation strategies. Prototype systems are typically monitored by expensive and bulky fluorescence microscopy systems and the development of integrated, direct electric detection of single filaments would strongly benefit applications and scale-up. We present estimates for the viability of such a detector by calculating the electrostatic potential change generated at a carbon nanotube transistor by a motile actin filament or microtubule under realistic gliding assay conditions. We combine this with detection limits based on previous state-of-the-art experiments using carbon nanotube transistors to detect catalysis by a bound lysozyme molecule and melting of a bound short-strand DNA molecule. Our results show that detection should be possible for both actin and microtubules using existing low ionic strength buffers given good device design, e.g., by raising the transistor slightly above the guiding channel floor. We perform studies as a function of buffer ionic strength, height of the transistor above the guiding channel floor, presence/absence of the casein surface passivation layer for microtubule assays and the linear charge density of the actin filaments/microtubules. We show that detection of microtubules is a more likely prospect given their smaller height of travel above the surface, higher negative charge density and the casein passivation, and may possibly be achieved with the nanoscale transistor sitting directly on the guiding channel floor. △ Less

Submitted 22 February, 2021; v1 submitted 15 February, 2021; originally announced February 2021.

Comments: Submitted to New Journal of Physics

Journal ref: New J. Phys. 23, 065003 (2021)

Fundamental energy cost of finite-time computing

Authors: Michael Konopik, Till Korten, Eric Lutz, Heiner Linke

Abstract: The fundamental energy cost of irreversible computing is given by the Landauer bound of $kT \ln2$~/bit. However, this limit is only achievable for infinite-time processes. We here determine the fundamental energy cost of finite-time irreversible computing \er{within the framework of nonequilibrium thermodynamics}. Comparing the lower bounds of energy required by ideal serial and parallel computers… ▽ More The fundamental energy cost of irreversible computing is given by the Landauer bound of $kT \ln2$~/bit. However, this limit is only achievable for infinite-time processes. We here determine the fundamental energy cost of finite-time irreversible computing \er{within the framework of nonequilibrium thermodynamics}. Comparing the lower bounds of energy required by ideal serial and parallel computers to solve a problem of a given size in a given finite time, we find that the energy cost of a serial computer fundamentally diverges with increasing problem size, whereas that of a parallel computer can stay close to the Landauer limit. We discuss the implications of this result in the context of current technology, and for different degrees of parallelization and amounts of overhead. Our findings provide a physical basis for the design of energy efficient computers. △ Less

Submitted 18 January, 2021; originally announced January 2021.

Journal ref: Nat Commun 14, 447 (2023)

Hot-Carrier Separation in Heterostructure Nanowires observed by Electron-Beam Induced Current

Authors: Jonatan Fast, Enrique Barrigon, Mukesh Kumar, Yang Chen, Lars Samuelson, Magnus Borgström, Anders Gustafsson, Steven Limpert, Adam Burke, Heiner Linke

Abstract: The separation of hot carriers in semiconductors is of interest for applications such as thermovoltaic photodetection and third-generation photovoltaics. Semiconductor nanowires offer several potential advantages for effective hot-carrier separation such as: a high degree of control and flexibility in heterostructure-based band engineering, increased hot-carrier temperatures compared to bulk, and… ▽ More The separation of hot carriers in semiconductors is of interest for applications such as thermovoltaic photodetection and third-generation photovoltaics. Semiconductor nanowires offer several potential advantages for effective hot-carrier separation such as: a high degree of control and flexibility in heterostructure-based band engineering, increased hot-carrier temperatures compared to bulk, and a geometry well suited for local control of light absorption. Indeed, InAs nanowires with a short InP energy barrier have been observed to produce electric power under global illumination, with an open-circuit voltage exceeding the Shockley-Queisser limit. To understand this behaviour in more detail, it is necessary to maintain control over the precise location of electron-hole pair-generation in the nanowire. In this work we perform electron-beam induced current measurements with high spatial resolution, and demonstrate the role of the InP barrier in extracting energetic electrons. We interprete the results in terms of hot-carrier separation, and extract estimates of the hot carrier mean free path. △ Less

Submitted 3 April, 2020; originally announced April 2020.

Implementing an insect brain computational circuit using III-V nanowire components in a single shared waveguide optical network

Authors: David O. Winge, Steven Limpert, Heiner Linke, Magnus T. Borgström, Barbara Webb, Stanley Heinze, Anders Mikkelsen

Abstract: Recent developments in photonics include efficient nanoscale optoelectronic components and novel methods for sub-wavelength light manipulation. Here, we explore the potential offered by such devices as a substrate for neuromorphic computing. We propose an artificial neural network in which the weighted connectivity between nodes is achieved by emitting and receiving overlapping light signals insid… ▽ More Recent developments in photonics include efficient nanoscale optoelectronic components and novel methods for sub-wavelength light manipulation. Here, we explore the potential offered by such devices as a substrate for neuromorphic computing. We propose an artificial neural network in which the weighted connectivity between nodes is achieved by emitting and receiving overlapping light signals inside a shared quasi 2D waveguide. This decreases the circuit footprint by at least an order of magnitude compared to existing optical solutions. The reception, evaluation and emission of the optical signals are performed by a neuron-like node constructed from known, highly efficient III-V nanowire optoelectronics. This minimizes power consumption of the network. To demonstrate the concept, we build a computational model based on an anatomically correct, functioning model of the central-complex navigation circuit of the insect brain. We simulate in detail the optical and electronic parts required to reproduce the connectivity of the central part of this network, using experimentally derived parameters. The results are used as input in the full model and we demonstrate that the functionality is preserved. Our approach points to a general method for drastically reducing the footprint and improving power efficiency of optoelectronic neural networks, leveraging the superior speed and energy efficiency of light as a carrier of information. △ Less

Submitted 2 March, 2020; originally announced March 2020.

Comments: 28 pages, 6 figures; supplementary information 15 pages, 8 figures

Journal ref: ACS Photonics 2020

Optimal power and efficiency of single quantum dot heat engines: theory and experiment

Authors: Martin Josefsson, Artis Svilans, Heiner Linke, Martin Leijnse

Abstract: Quantum dots (QDs) can serve as near perfect energy filters and are therefore of significant interest for the study of thermoelectric energy conversion close to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano. 13, 920 (2018)] realized a QD heat engine with performance near these limits and in excellent agreement with theoretical predictions. However, these experiments als… ▽ More Quantum dots (QDs) can serve as near perfect energy filters and are therefore of significant interest for the study of thermoelectric energy conversion close to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano. 13, 920 (2018)] realized a QD heat engine with performance near these limits and in excellent agreement with theoretical predictions. However, these experiments also highlighted a need for more theory to help guide and understand the practical optimization of QD heat engines, in particular regarding the role of tunnel couplings on the performance at maximum power and efficiency for QDs that couple seemingly weakly to electronic reservoirs. Furthermore, these experiments also highlighted the critical role of the external load when optimizing the performance of a QD heat engine in practice. To provide further insight into the operation of these engines we use the Anderson impurity model together with a Master equation approach to perform power and efficiency calculations up to co-tunneling order. This is combined with additional thermoelectric experiments on a QD embedded in a nanowire where the power is measured using two methods. We use the measurements to present an experimental procedure for efficiently finding the external load $R_P$ which should be connected to the engine to optimize power output. Our theoretical estimates of $R_P$ show a good agreement with the experimental results, and we show that second order tunneling processes and non-linear effects have little impact close to maximum power, allowing us to derive a simple analytic expression for $R_P$. In contrast, we find that the electron contribution to the thermoelectric efficiency is significantly reduced by second order tunneling processes, even for rather weak tunnel couplings. △ Less

Submitted 5 July, 2019; v1 submitted 29 March, 2019; originally announced March 2019.

Journal ref: Phys. Rev. B 99, 235432 (2019)

Achieving short high-quality gate-all-around structures for horizontal nanowire field-effect transistors

Authors: J. G. Gluschke, J. Seidl, A. M. Burke, R. W. Lyttleton, D. J. Carrad, A. R. Ullah, S. Fahlvik Svensson, S. Lehmann, H. Linke, A. P. Micolich

Abstract: We introduce a fabrication method for gate-all-around nanowire field-effect transistors. Single nanowires were aligned perpendicular to underlying bottom gates using a resist-trench alignment technique. Top gates were then defined aligned to the bottom gates to form gate-all-around structures. This approach overcomes significant limitations in minimal obtainable gate length and gate-length control… ▽ More We introduce a fabrication method for gate-all-around nanowire field-effect transistors. Single nanowires were aligned perpendicular to underlying bottom gates using a resist-trench alignment technique. Top gates were then defined aligned to the bottom gates to form gate-all-around structures. This approach overcomes significant limitations in minimal obtainable gate length and gate-length control in previous horizontal wrap-gated nanowire transistors that arise because the gate is defined by wet etching. In the method presented here gate-length control is limited by the resolution of the electron-beam-lithography process. We demonstrate the versatility of our approach by fabricating a device with an independent bottom gate, top gate, and gate-all-around structure as well as a device with three independent gate-all-around structures with 300 nm, 200 nm, and 150 nm gate length. Our method enables us to achieve sub-threshold swings as low as 38 mV/dec at 77 K for a 150 nm gate length. △ Less

Submitted 8 October, 2018; originally announced October 2018.

Comments: Submitted to Nanotechnology

Thermoelectric characterization of the Kondo resonance in nanowire quantum dots

Authors: Artis Svilans, Martin Josefsson, Adam M. Burke, Sofia Fahlvik, Claes Thelander, Heiner Linke, Martin Leijnse

Abstract: We experimentally verify hitherto untested theoretical predictions about the thermoelectric properties of Kondo correlated quantum dots (QDs). The specific conditions required for this study are obtained by using QDs epitaxially grown in nanowires, combined with a recently developed method for controlling and measuring temperature differences at the nanoscale. This makes it possible to obtain data… ▽ More We experimentally verify hitherto untested theoretical predictions about the thermoelectric properties of Kondo correlated quantum dots (QDs). The specific conditions required for this study are obtained by using QDs epitaxially grown in nanowires, combined with a recently developed method for controlling and measuring temperature differences at the nanoscale. This makes it possible to obtain data of very high quality both below and above the Kondo temperature, and allows a quantitative comparison with theoretical predictions. Specifically, we verify that Kondo correlations can induce a polarity change of the thermoelectric current, which can be reversed either by increasing the temperature or by applying a magnetic field. △ Less

Submitted 20 July, 2018; originally announced July 2018.

Journal ref: Phys. Rev. Lett. 121, 206801 (2018)

A quantum-dot heat engine operating close to the thermodynamic efficiency limits

Authors: Martin Josefsson, Artis Svilans, Adam M. Burke, Eric A. Hoffmann, Sofia Fahlvik, Claes Thelander, Martin Leijnse, Heiner Linke

Abstract: Cyclical heat engines are a paradigm of classical thermodynamics, but are impractical for miniaturization because they rely on moving parts. A more recent concept is particle-exchange (PE) heat engines, which uses energy filtering to control a thermally driven particle flow between two heat reservoirs. As they do not require moving parts and can be realized in solid-state materials, they are suita… ▽ More Cyclical heat engines are a paradigm of classical thermodynamics, but are impractical for miniaturization because they rely on moving parts. A more recent concept is particle-exchange (PE) heat engines, which uses energy filtering to control a thermally driven particle flow between two heat reservoirs. As they do not require moving parts and can be realized in solid-state materials, they are suitable for low-power applications and miniaturization. It was predicted that PE engines could reach the same thermodynamically ideal efficiency limits as those accessible to cyclical engines, but this prediction has not been verified experimentally. Here, we demonstrate a PE heat engine based on a quantum dot (QD) embedded into a semiconductor nanowire. We directly measure the engine's steady-state electric power output and combine it with the calculated electronic heat flow to determine the electronic efficiency $η$. We find that at the maximum power conditions, $η$ is in agreement with the Curzon-Ahlborn efficiency and that the overall maximum $η$ is in excess of 70$\%$ of the Carnot efficiency while maintaining a finite power output. Our results demonstrate that thermoelectric power conversion can, in principle, be achieved close to the thermodynamic limits, with direct relevance for future hot-carrier photovoltaics, on-chip coolers or energy harvesters for quantum technologies. △ Less

Submitted 17 July, 2018; v1 submitted 2 October, 2017; originally announced October 2017.

Journal ref: Nature Nanotechnology 13, 920-924 (2018)

Thermoelectric power factor limit of a 1D nanowire

Authors: I-Ju Chen, Adam Burke, Artis Svilans, Heiner Linke, Claes Thelander

Abstract: In the past decade, there has been significant interest in the potentially advantageous thermoelectric properties of one-dimensional (1D) nanowires, but it has been challenging to find high thermoelectric power factors based on 1D effect in practice. Here we point out that there is an upper limit to the thermoelectric power factor of non-ballistic 1D nanowires, as a consequence of the recently est… ▽ More In the past decade, there has been significant interest in the potentially advantageous thermoelectric properties of one-dimensional (1D) nanowires, but it has been challenging to find high thermoelectric power factors based on 1D effect in practice. Here we point out that there is an upper limit to the thermoelectric power factor of non-ballistic 1D nanowires, as a consequence of the recently established quantum bound of thermoelectric power output. We experimentally test this limit in quasi-ballistic InAs nanowires by extracting the maximum power factor of the first 1D subband through I-V characterization, finding that the measured maximum power factors conform to the theoretical limit. The established limit predicts that a competitive power factor, on the order of mW/m-K^2, can be achieved by a single 1D electronic channel in state-of-the-art semiconductor nanowires with small cross-section and high crystal quality. △ Less

Submitted 29 September, 2017; originally announced September 2017.

Journal ref: Phys. Rev. Lett. 120, 177703 (2018)

Intersubband Quantum Disc-in-Nanowire Photodetectors with Normal-incidence Response in the Long-wavelength Infrared

Authors: Mohammad Karimi, Magnus Heurlin, Steven Limpert, Vishal Jain, Xulu Zeng, Irene Geijselaers, Ali Nowzari, Ying Fu, Lars Samuelson, Heiner Linke, Magnus T. Borgstrom, Hakan Pettersson

Abstract: Semiconductor nanowires offer great potential for realizing broadband photodetectors that are compatible with silicon technology. However, the spectral range of such detectors has so far been limited to selected regions in the ultraviolet, visible and near infrared. Here, we report on broadband nanowire heterostructure array photodetectors exhibiting a photoresponse from the visible to long-wavele… ▽ More Semiconductor nanowires offer great potential for realizing broadband photodetectors that are compatible with silicon technology. However, the spectral range of such detectors has so far been limited to selected regions in the ultraviolet, visible and near infrared. Here, we report on broadband nanowire heterostructure array photodetectors exhibiting a photoresponse from the visible to long-wavelength infrared. In particular, the infrared response from 3-20 um is enabled by normal incidence excitation of intersubband transitions in low-bandgap InAsP quantum discs synthesized axially within InP nanowires. The optical characteristics are explained by the excitation of the longitudinal component of optical modes in the photonic crystal formed by the nanostructured portion of the detectors, combined with a non-symmetric potential profile of the discs resulting from synthesis. Our results provide a generalizable insight into how broadband nanowire photodetectors may be designed, and how engineered nanowire heterostructures open up new fascinating opportunities for optoelectronics. △ Less

Submitted 10 September, 2017; originally announced September 2017.

Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage

Authors: Steven Limpert, Adam Burke, I-Ju Chen, Nicklas Anttu, Sebastian Lehmann, Sofia Fahlvik, Stephen Bremner, Gavin Conibeer, Claes Thelander, Mats-Erik Pistol, Heiner Linke

Abstract: Compared to traditional pn-junction photovoltaics, hot carrier solar cells offer potentially higher efficiency by extracting work from the kinetic energy of photogenerated "hot carriers" before they cool to the lattice temperature. Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap materials, where… ▽ More Compared to traditional pn-junction photovoltaics, hot carrier solar cells offer potentially higher efficiency by extracting work from the kinetic energy of photogenerated "hot carriers" before they cool to the lattice temperature. Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap materials, where the potential efficiency gain is highest. Recently, a high open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire with a low bandgap of 0.39 eV, and was interpreted in terms of a photothermoelectric effect. Here, we point out that this device is a hot carrier solar cell and discuss its performance in those terms. In the demonstrated devices, InP heterostructures are used as energy filters in order to thermoelectrically harvest the energy of hot electrons photogenerated in InAs absorber segments. The obtained photovoltage depends on the heterostructure design of the energy filter and is therefore tunable. By using a high-resistance, thermionic barrier an open-circuit voltage is obtained that is in excess of the Shockley-Queisser limit. These results provide generalizable insight into how to realize high voltage hot carrier solar cells in low-bandgap materials, and therefore are a step towards the demonstration of higher efficiency hot carrier solar cells. △ Less

Submitted 16 August, 2017; originally announced August 2017.

Experiments on thermoelectric properties of quantum dots

Authors: Artis Svilans, Martin Leijnse, Heiner Linke

Abstract: Quantum dots (QDs) are good model systems for fundamental studies of mesoscopic transport phenomena using thermoelectric effects because of their small size, electrostatically tunable properties and thermoelectric response characteristics that are very sensitive to small thermal biases. Here we provide a review of experimental studies on thermoelectric properties of single QDs realized in two-dime… ▽ More Quantum dots (QDs) are good model systems for fundamental studies of mesoscopic transport phenomena using thermoelectric effects because of their small size, electrostatically tunable properties and thermoelectric response characteristics that are very sensitive to small thermal biases. Here we provide a review of experimental studies on thermoelectric properties of single QDs realized in two-dimensional electron gases, single-walled carbon nanotubes and semiconductor nanowires. A key requirement for such experiments is methods for nanoscale thermal biasing. We briefly review the main techniques used in the field, namely, heating of the QD contacts, side heating and top heating, and touch upon their relative advantages. The thermoelectric response of a QD as a function of gate potential has a characteristic oscillatory behavior with the same period as is observed for conductance peaks. Much of the existing literature focuses on the agreement between experiments and theory, particularly for amplitude and line-shape of the thermovoltage $V_{th}$. A general observation is that the widely used single-electron tunneling approximation for QDs has limited success in reproducing measured $V_{th}$. Landauer-type calculations are often found to describe measurement results better despite the large electron-electron interactions in QDs. More recently, nonlinear thermoelectric effects have moved into the focus of attention and we offer a brief overview of experiments done so far. We conclude by discussing open questions and avenues for future work, including the role of asymmetries in tunnel- and capacitive couplings in thermoelectric behavior of QDs. △ Less

Submitted 14 November, 2016; originally announced November 2016.

Journal ref: Comptes rendus - Physique 17 (2016) pp. 1096-1108

Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot

Authors: Artis Svilans, Adam M. Burke, Sofia Fahlvik Svensson, Martin Leijnse, Heiner Linke

Abstract: Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been cha… ▽ More Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been challenging to observe because, first, it is experimentally difficult to apply a sufficiently large thermal bias at the nanoscale and, second, it is difficult to distinguish thermal bias effects from purely temperature-dependent effects due to overall heating of a device. Here we take advantage of a novel thermal biasing technique and demonstrate a nonlinear thermoelectric response in a quantum dot which is defined in a heterostructured semiconductor nanowire. We also show that a theoretical model based on the Master equations fully explains the observed nonlinear thermoelectric response given the energy-dependent transport properties of the quantum dot. △ Less

Submitted 28 October, 2015; originally announced October 2015.

Comments: Cite as: A. Svilans, et al., Physica E (2015), http://dx.doi.org/10.1016/j.physe.2015.10.007

Journal ref: Physica E 82 (2015) 34-38

Reversible electron-hole separation in a hot carrier solar cell

Authors: Steven Limpert, Stephen Bremner, Heiner Linke

Abstract: Hot-carrier solar cells are envisioned to utilize energy filtering to extract power from photogenerated electron-hole pairs before they thermalize with the lattice, and thus potentially offer higher power conversion efficiency compared to conventional, single absorber solar cells. The efficiency of hot-carrier solar cells can be expected to strongly depend on the details of the energy filtering pr… ▽ More Hot-carrier solar cells are envisioned to utilize energy filtering to extract power from photogenerated electron-hole pairs before they thermalize with the lattice, and thus potentially offer higher power conversion efficiency compared to conventional, single absorber solar cells. The efficiency of hot-carrier solar cells can be expected to strongly depend on the details of the energy filtering process, a relationship which to date has not been satisfactorily explored. Here, we establish the conditions under which electron-hole separation in hot-carrier solar cells can occur reversibly, that is, at maximum energy conversion efficiency. We thus focus our analysis on the internal operation of the hot-carrier solar cell itself, and in this work do not consider the photon-mediated coupling to the sun. After deriving an expression for the voltage of a hot-carrier solar cell valid under conditions of both reversible and irreversible electrical operation, we identify separate contributions to the voltage from the thermoelectric effect and the photovoltaic effect. We find that, under specific conditions, the energy conversion efficiency of a hot-carrier solar cell can exceed the Carnot limit set by the intra-device temperature gradient alone, due to the additional contribution of the quasi-Fermi level splitting in the absorber. We also establish that the open-circuit voltage of a hot-carrier solar cell is not limited by the band gap of the absorber, due to the additional thermoelectric contribution to the voltage. Additionally, we find that a hot-carrier solar cell can be operated in reverse as a thermally driven solid-state light emitter. Our results help explore the fundamental limitations of hot-carrier solar cells, and provide a first step towards providing experimentalists with a guide to the optimal configuration of devices. △ Less

Submitted 16 September, 2015; originally announced September 2015.

Comments: 31 pages, 5 figures

Journal ref: New J. Phys. 17 095004 (2015)

InAs nanowire transistors with multiple, independent wrap-gate segments

Authors: A. M. Burke, D. J. Carrad, J. G. Gluschke, K. Storm, S. Fahlvik Svensson, H. Linke, L. Samuelson, A. P. Micolich

Abstract: We report a method for making horizontal wrap-gate nanowire transistors with up to four independently controllable wrap-gated segments. While the step up to two independent wrap-gates requires a major change in fabrication methodology, a key advantage to this new approach, and the horizontal orientation more generally, is that achieving more than two wrap-gate segments then requires no extra fabri… ▽ More We report a method for making horizontal wrap-gate nanowire transistors with up to four independently controllable wrap-gated segments. While the step up to two independent wrap-gates requires a major change in fabrication methodology, a key advantage to this new approach, and the horizontal orientation more generally, is that achieving more than two wrap-gate segments then requires no extra fabrication steps. This is in contrast to the vertical orientation, where a significant subset of the fabrication steps needs to be repeated for each additional gate. We show that cross-talk between adjacent wrap-gate segments is negligible despite separations less than 200 nm. We also demonstrate the ability to make multiple wrap-gate transistors on a single nanowire using the exact same process. The excellent scalability potential of horizontal wrap-gate nanowire transistors makes them highly favourable for the development of advanced nanowire devices and possible integration with vertical wrap-gate nanowire transistors in 3D nanowire network architectures. △ Less

Submitted 7 May, 2015; originally announced May 2015.

Comments: 18 pages, 5 figures, In press for Nano Letters (DOI below)

Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics

Authors: Sofia Fahlvik Svensson, Adam M. Burke, Damon J. Carrad, Martin Leijnse, Heiner Linke, Adam P. Micolich

Abstract: We use the strongly temperature-dependent ionic mobility in polymer electrolytes to 'freeze in' specific ionic charge environments around a nanowire using a local wrap-gate geometry. This enables us to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 200 Kelvin, which we characterize in detail by combining conductance and… ▽ More We use the strongly temperature-dependent ionic mobility in polymer electrolytes to 'freeze in' specific ionic charge environments around a nanowire using a local wrap-gate geometry. This enables us to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 200 Kelvin, which we characterize in detail by combining conductance and thermovoltage measurements with modeling. Our results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications. △ Less

Submitted 11 November, 2014; originally announced November 2014.

Comments: Published in Advanced Functional Materials. Includes colour versions of figures and supplementary information

Thermoelectric performance of classical topological insulator nanowires

Authors: Johannes Gooth, Jan Goeran Gluschke, Robert Zierold, Martin Leijnse, Heiner Linke, Kornelius Nielsch

Abstract: There is currently substantial effort being invested into creating efficient thermoelectric nanowires based on topological insulator chalcogenide-type materials. A key premise of these efforts is the assumption that the generally good thermoelectric properties that these materials exhibit in bulk form will translate into similarly good or even better thermoelectric performance of the same material… ▽ More There is currently substantial effort being invested into creating efficient thermoelectric nanowires based on topological insulator chalcogenide-type materials. A key premise of these efforts is the assumption that the generally good thermoelectric properties that these materials exhibit in bulk form will translate into similarly good or even better thermoelectric performance of the same materials in nanowire form. Here, we calculate thermoelectric performance of topological insulator nanowires based on Bi2Te3, Sb2Te3 and Bi2Se3 as a function of diameter and Fermi level. We show that the thermoelectric performance of topological insulator nanowires does not derive from the properties of the bulk material in a straightforward way. For all investigated systems the competition between surface states and bulk channel causes a significant modification of the thermoelectric transport coefficients if the diameter is reduced into the sub-10 um range. Key aspects are that the surface and bulk states are optimized at different Fermi levels or have different polarity as well as the high surface to volume ratio of the nanowires. This limits the maximum thermoelectric performance of topological insulator nanowires and thus their application in efficient thermoelectric devices. △ Less

Submitted 20 April, 2015; v1 submitted 7 May, 2014; originally announced May 2014.

Electron-beam patterning of polymer electrolyte films to make multiple nanoscale gates for nanowire transistors

Authors: D. J. Carrad, A. M. Burke, R. W. Lyttleton, H. J. Joyce, H. H. Tan, C. Jagadish, K. Storm, H. Linke, L. Samuelson, A. P. Micolich

Abstract: We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO$_{4}$ polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap which reduces… ▽ More We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO$_{4}$ polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap which reduces parasitic effects and enables multiple, independently controllable gates. The method's simplicity broadens the scope for using polymer electrolyte gating in studies of nanowires and other nanoscale devices. △ Less

Submitted 7 April, 2014; originally announced April 2014.

Comments: 20 pages, 4 figures. Supplementary information available at Nano Letters or by contact with APM

Journal ref: Nano Letters 14, 94 (2014)

Nonlinear thermovoltage and thermocurrent in quantum dots

Authors: Sofia Fahlvik Svensson, Eric A Hoffmann, Natthapon Nakpathomkun, Phillip M Wu, Hongqi Xu, Henrik A Nilsson, David Sánchez, Vyacheslavs Kashcheyevs, Heiner Linke

Abstract: Quantum dots are model systems for quantum thermoelectric behavior because of the ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, w… ▽ More Quantum dots are model systems for quantum thermoelectric behavior because of the ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, we measure thermovoltage and thermocurrent that are strongly nonlinear in the applied thermal bias. We show that most of the observed nonlinear effects can be understood in terms of a renormalization of the quantum-dot energy levels as a function of applied thermal bias and provide a theoretical model of the nonlinear thermovoltage taking renormalization into account. Furthermore, we propose a theory that explains a possible source of the observed, pronounced renormalization effect by the melting of Kondo correlations in the mixed-valence regime. The ability to control nonlinear thermoelectric behavior expands the range in which quantum thermoelectric effects may be used for efficient energy conversion. △ Less

Submitted 2 July, 2013; originally announced July 2013.

Comments: 14 pages, 5 figures

Journal ref: New J. Phys.15, 105011 (2013)

Experimental verification of reciprocity relations in quantum thermoelectric transport

Authors: J. Matthews, F. Battista, D. Sanchez, P. Samuelsson, H. Linke

Abstract: Symmetry relations are manifestations of fundamental principles and constitute cornerstones of modern physics. An example are the Onsager relations between coefficients connecting thermodynamic fluxes and forces, central to transport theory and experiments. Initially formulated for classical systems, these reciprocity relations are also fulfilled in quantum conductors. Surprisingly, novel relation… ▽ More Symmetry relations are manifestations of fundamental principles and constitute cornerstones of modern physics. An example are the Onsager relations between coefficients connecting thermodynamic fluxes and forces, central to transport theory and experiments. Initially formulated for classical systems, these reciprocity relations are also fulfilled in quantum conductors. Surprisingly, novel relations have been predicted specifically for thermoelectric transport. However, whereas these thermoelectric reciprocity relations have to date not been verified, they have been predicted to be sensitive to inelastic scattering, always present at finite temperature. The question whether the relations exist in practice is important for thermoelectricity: whereas their existence may simplify the theory of complex thermoelectric materials, their absence has been shown to enable, in principle, higher thermoelectric energy conversion efficiency for a given material quality. Here we experimentally verify the thermoelectric reciprocity relations in a four-terminal mesoscopic device where each terminal can be electrically and thermally biased, individually. The linear response thermoelectric coefficients are found to be symmetric under simultaneous reversal of magnetic field and exchange of injection and emission contacts. Intriguingly, we also observe the breakdown of the reciprocity relations as a function of increasing thermal bias. Our measurements thus clearly establish the existence of the thermoelectric reciprocity relations, as well as the possibility to control their breakdown with the potential to enhance thermoelectric performance △ Less

Submitted 27 October, 2014; v1 submitted 16 June, 2013; originally announced June 2013.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 90, 165428 (2014)

Heat flow in InAs/InP heterostructure nanowires

Authors: Jason Matthews, Eric A. Hoffmann, Carsten Weber, Andreas Wacker, Heiner Linke

Abstract: The transfer of heat between electrons and phonons plays a key role for thermal management in future nanowire-based devices, but only a few experimental measurements of electron-phonon (e-ph) coupling in nanowires are available. Here, we combine experimental temperature measurements on an InAs/InP heterostructure nanowire system with finite element modeling (FEM) to extract information on heat flo… ▽ More The transfer of heat between electrons and phonons plays a key role for thermal management in future nanowire-based devices, but only a few experimental measurements of electron-phonon (e-ph) coupling in nanowires are available. Here, we combine experimental temperature measurements on an InAs/InP heterostructure nanowire system with finite element modeling (FEM) to extract information on heat flow mediated by e-ph coupling. We find that the electron and phonon temperatures in our system are highly coupled even at temperatures as low as 2 K. Additionally, we find evidence that the usual power-law temperature dependence of electron-phonon coupling may not correctly describe the coupling in nanowires and show that this result is consistent with previous research on similar one-dimensional electron systems. We also compare the strength of the observed e-ph coupling to a theoretical analysis of e-ph interaction in InAs nanowires, which predicts a significantly weaker coupling strength than observed experimentally. △ Less

Submitted 21 September, 2012; originally announced September 2012.

Comments: 9 pages, 6 figures

Journal ref: Physical Review B 86, 174302 (2012)

Is it the boundaries or disorder that dominates electron transport in semiconductor `billiards'?

Authors: A. P. Micolich, A. M. See, B. C. Scannell, C. A. Marlow, T. P. Martin, I. Pilgrim, A. R. Hamilton, H. Linke, R. P. Taylor

Abstract: Semiconductor billiards are often considered as ideal systems for studying dynamical chaos in the quantum mechanical limit. In the traditional picture, once the electron's mean free path, as determined by the mobility, becomes larger than the device, disorder is negligible and electron trajectories are shaped by specular reflection from the billiard walls alone. Experimental insight into the elect… ▽ More Semiconductor billiards are often considered as ideal systems for studying dynamical chaos in the quantum mechanical limit. In the traditional picture, once the electron's mean free path, as determined by the mobility, becomes larger than the device, disorder is negligible and electron trajectories are shaped by specular reflection from the billiard walls alone. Experimental insight into the electron dynamics is normally obtained by magnetoconductance measurements. A number of recent experimental studies have shown these measurements to be largely independent of the billiards exact shape, and highly dependent on sample-to-sample variations in disorder. In this paper, we discuss these more recent findings within the full historical context of work on semiconductor billiards, and offer strong evidence that small-angle scattering at the sub-100 nm length-scale dominates transport in these devices, with important implications for the role these devices can play for experimental tests of ideas in quantum chaos. △ Less

Submitted 22 April, 2012; originally announced April 2012.

Comments: Submitted to Fortschritte der Physik for special issue on Quantum Physics with Non-Hermitian Operators

Lineshape of the thermopower of quantum dots

Authors: S. Fahlvik Svensson, A. I. Persson, E. A. Hoffmann, N. Nakpathomkun, H. A. Nilsson, H. Q. Xu, L. Samuelson, H. Linke

Abstract: Quantum dots are an important model system for thermoelectric phenomena, and may be used to enhance the thermal-to-electric energy conversion efficiency in functional materials. It is therefore important to obtain a detailed understanding of a quantum-dot's thermopower as a function of the Fermi energy. However, so far it has proven difficult to take effects of co-tunnelling into account in the in… ▽ More Quantum dots are an important model system for thermoelectric phenomena, and may be used to enhance the thermal-to-electric energy conversion efficiency in functional materials. It is therefore important to obtain a detailed understanding of a quantum-dot's thermopower as a function of the Fermi energy. However, so far it has proven difficult to take effects of co-tunnelling into account in the interpretation of experimental data. Here we show that a single-electron tunnelling model, using knowledge of the dot's electrical conductance which in fact includes all-order co-tunneling effects, predicts the thermopower of quantum dots as a function of the relevant energy scales, in very good agreement with experiment. △ Less

Submitted 3 October, 2011; originally announced October 2011.

Comments: 10 pages, 5 figures

Mesoscopic Thermovoltage Measurement Design

Authors: E. A. Hoffmann, H. A. Nilsson, L. Samuelson, H. Linke

Abstract: Quantitative thermoelectric measurements in the mesoscopic regime require accurate knowledge of temperature, thermovoltage, and device energy scales. We consider the effect of a finite load resistance on thermovoltage measurements of InAs/InP heterostructure nanowires. Load resistance and ac attenuation distort the measured thermovoltage therefore complicating the evaluation of device performance.… ▽ More Quantitative thermoelectric measurements in the mesoscopic regime require accurate knowledge of temperature, thermovoltage, and device energy scales. We consider the effect of a finite load resistance on thermovoltage measurements of InAs/InP heterostructure nanowires. Load resistance and ac attenuation distort the measured thermovoltage therefore complicating the evaluation of device performance. Understanding these effects improves experimental design and data interpretation. △ Less

Submitted 5 September, 2011; originally announced September 2011.

Comments: 2 pages, 3 figures

Thermally driven ballistic rectifer

Authors: Jason Matthews, David Sánchez, Marcus Larsson, Heiner Linke

Abstract: The response of electric devices to an applied thermal gradient has, so far, been studied almost exclusively in two-terminal devices. Here we present measurements of the response to a thermal bias of a four-terminal, quasi-ballistic junction with a central scattering site. We find a novel transverse thermovoltage measured across isothermal contacts. Using a multi-terminal scattering model extended… ▽ More The response of electric devices to an applied thermal gradient has, so far, been studied almost exclusively in two-terminal devices. Here we present measurements of the response to a thermal bias of a four-terminal, quasi-ballistic junction with a central scattering site. We find a novel transverse thermovoltage measured across isothermal contacts. Using a multi-terminal scattering model extended to the weakly non-linear voltage regime, we show that the device's response to a thermal bias can be predicted from its nonlinear response to an electric bias. Our approach forms a foundation for the discovery and understanding of advanced, nonlocal, thermoelectric phenomena that in the future may lead to novel thermoelectric device concepts. △ Less

Submitted 30 April, 2012; v1 submitted 15 July, 2011; originally announced July 2011.

Comments: 4 pages, 4 figures; minor to moderate clarifications and simplifications in v3

Journal ref: Phys. Rev. B 85, 205309 (2012)

Increasing thermoelectric performance using coherent transport

Authors: Olov Karlstrom, Heiner Linke, Gunnar Karlstrom, Andreas Wacker

Abstract: We show that coherent electron transport through zero-dimensional systems can be used to tailor the shape of the system's transmission function. This quantum-engineering approach can be used to enhance the performance of quantum dots or molecules in thermal-to-electric power conversion. Specifically, we show that electron interference in a two-level system can substantially improve the maximum the… ▽ More We show that coherent electron transport through zero-dimensional systems can be used to tailor the shape of the system's transmission function. This quantum-engineering approach can be used to enhance the performance of quantum dots or molecules in thermal-to-electric power conversion. Specifically, we show that electron interference in a two-level system can substantially improve the maximum thermoelectric power and the efficiency at maximum power by suppressing parasitic charge flow near the Fermi energy, and by reducing electronic heat conduction. We discuss possible realizations of this approach in molecular junctions or quantum dots. △ Less

Submitted 7 October, 2011; v1 submitted 4 July, 2011; originally announced July 2011.

Comments: 4+ pages, 4 figures

Journal ref: Physical Review B 84, 113415 (2011)

Probing the Sensitivity of Electron Wave Interference to Disorder-Induced Scattering in Solid-State Devices

Authors: B. C. Scannell, I. Pilgrim, A. M. See, R. D. Montgomery, P. K. Morse, M. S. Fairbanks, C. A. Marlow, H. Linke, I. Farrer, D. A. Ritchie, A. R. Hamilton, A. P. Micolich, L. Eaves, R. P. Taylor

Abstract: The study of electron motion in semiconductor billiards has elucidated our understanding of quantum interference and quantum chaos. The central assumption is that ionized donors generate only minor perturbations to the electron trajectories, which are determined by scattering from billiard walls. We use magnetoconductance fluctuations as a probe of the quantum interference and show that these fluc… ▽ More The study of electron motion in semiconductor billiards has elucidated our understanding of quantum interference and quantum chaos. The central assumption is that ionized donors generate only minor perturbations to the electron trajectories, which are determined by scattering from billiard walls. We use magnetoconductance fluctuations as a probe of the quantum interference and show that these fluctuations change radically when the scattering landscape is modified by thermally-induced charge displacement between donor sites. Our results challenge the accepted understanding of quantum interference effects in nanostructures. △ Less

Submitted 7 May, 2012; v1 submitted 28 June, 2011; originally announced June 2011.

Comments: 8 pages, 5 figures, Submitted to Physical Review B

Signatures of Wigner Localization in Epitaxially Grown Nanowires

Authors: L. H. Kristinsdóttir, J. C. Cremon, H. A. Nilsson, H. Q. Xu, L. Samuelson, H. Linke, A. Wacker, S. M. Reimann

Abstract: It was predicted by Wigner in 1934 that the electron gas will undergo a transition to a crystallized state when its density is very low. Whereas significant progress has been made towards the detection of electronic Wigner states, their clear and direct experimental verification still remains a challenge. Here we address signatures of Wigner molecule formation in the transport properties of InSb n… ▽ More It was predicted by Wigner in 1934 that the electron gas will undergo a transition to a crystallized state when its density is very low. Whereas significant progress has been made towards the detection of electronic Wigner states, their clear and direct experimental verification still remains a challenge. Here we address signatures of Wigner molecule formation in the transport properties of InSb nanowire quantum dot systems, where a few electrons may form localized states depending on the size of the dot (i.e. the electron density). By a configuration interaction approach combined with an appropriate transport formalism, we are able to predict the transport properties of these systems, in excellent agreement with experimental data. We identify specific signatures of Wigner state formation, such as the strong suppression of the antiferromagnetic coupling, and are able to detect the onset of Wigner localization, both experimentally and theoretically, by studying different dot sizes. △ Less

Submitted 31 January, 2011; v1 submitted 25 October, 2010; originally announced October 2010.

Comments: 4 pages, 4 figures

Journal ref: Physical Review B 83, 041101 (R) (2011)

Thermoelectric efficiency at maximum power in low-dimensional systems

Authors: Natthapon Nakpathomkun, Hongqi Q. Xu, Heiner Linke

Abstract: Low-dimensional electronic systems in thermoelectrics have the potential to achieve high thermal-to-electric energy conversion efficiency. A key measure of performance is the efficiency when the device is operated under maximum power conditions. Here we study the efficiency at maximum power of three low-dimensional, thermoelectric systems: a zero-dimensional quantum dot (QD) with a Lorentzian tran… ▽ More Low-dimensional electronic systems in thermoelectrics have the potential to achieve high thermal-to-electric energy conversion efficiency. A key measure of performance is the efficiency when the device is operated under maximum power conditions. Here we study the efficiency at maximum power of three low-dimensional, thermoelectric systems: a zero-dimensional quantum dot (QD) with a Lorentzian transmission resonance of finite width, a one-dimensional (1D) ballistic conductor, and a thermionic (TI) power generator formed by a two-dimensional energy barrier. In all three systems, the efficiency at maximum power is independent of temperature, and in each case a careful tuning of relevant energies is required to achieve maximal performance. We find that quantum dots perform relatively poorly under maximum power conditions, with relatively low efficiency and small power throughput. Ideal one-dimensional conductors offer the highest efficiency at maximum power (36% of the Carnot efficiency). Whether 1D or TI systems achieve the larger maximum power output depends on temperature and area filling factor. These results are also discussed in the context of the traditional figure of merit $ZT$. △ Less

Submitted 7 October, 2010; originally announced October 2010.

A Master equation approach to modeling an artificial protein motor

Authors: Nathan J. Kuwada, Gerhard A. Blab, Heiner Linke

Abstract: Linear bio-molecular motors move unidirectionally along a track by coordinating several different processes, such as fuel (ATP) capture, hydrolysis, conformational changes, binding and unbinding from a track, and center-of-mass diffusion. A better understanding of the interdependencies between these processes, which take place over a wide range of different time scales, would help elucidate the ge… ▽ More Linear bio-molecular motors move unidirectionally along a track by coordinating several different processes, such as fuel (ATP) capture, hydrolysis, conformational changes, binding and unbinding from a track, and center-of-mass diffusion. A better understanding of the interdependencies between these processes, which take place over a wide range of different time scales, would help elucidate the general operational principles of molecular motors. Artificial molecular motors present a unique opportunity for such a study because motor structure and function are a priori known. Here we describe use of a Master equation approach, integrated with input from Langevin and molecular dynamics modeling, to stochastically model a molecular motor across many time scales. We apply this approach to a specific concept for an artificial protein motor, the Tumbleweed. △ Less

Submitted 7 April, 2010; originally announced April 2010.

Comments: Submitted to Chemical Physics; 9 pages, 7 figures

Measuring Temperature Gradients over Nanometer Length Scales

Authors: E. A. Hoffmann, H. A. Nilsson, J. E. Matthews, N. Nakpathomkun, A. I. Persson, L. Samuelson, H. Linke

Abstract: When a quantum dot is subjected to a thermal gradient, the temperature of electrons entering the dot can be determined from the dot's thermocurrent if the conductance spectrum and background temperature are known. We demonstrate this technique by measuring the temperature difference across a 15 nm quantum dot embedded in a nanowire. This technique can be used when the dot's energy states are sep… ▽ More When a quantum dot is subjected to a thermal gradient, the temperature of electrons entering the dot can be determined from the dot's thermocurrent if the conductance spectrum and background temperature are known. We demonstrate this technique by measuring the temperature difference across a 15 nm quantum dot embedded in a nanowire. This technique can be used when the dot's energy states are separated by many kT and will enable future quantitative investigations of electron-phonon interaction, nonlinear thermoelectric effects, and the effciency of thermoelectric energy conversion in quantum dots. △ Less

Submitted 15 January, 2009; originally announced January 2009.

Comments: 6 pages, 5 figures

Journal ref: Nano Lett., 2009, 9 (2), pp 779-783

Realization of a feedback controlled flashing ratchet

Authors: Benjamin J. Lopez, Nathan J. Kuwada, Erin M. Craig, Brian R. Long, Heiner Linke

Abstract: A flashing ratchet transports diffusive particles using a time-dependent, asymmetric potential. Particle speed is predicted to increase when a feedback algorithm based on particle positions is used. We have experimentally realized such a feedback ratchet using an optical line trap, and observed that use of feedback increases velocity by up to an order of magnitude. We compare two different feedb… ▽ More A flashing ratchet transports diffusive particles using a time-dependent, asymmetric potential. Particle speed is predicted to increase when a feedback algorithm based on particle positions is used. We have experimentally realized such a feedback ratchet using an optical line trap, and observed that use of feedback increases velocity by up to an order of magnitude. We compare two different feedback algorithms for small particle numbers, and find good agreement with simulations. We also find that existing algorithms can be improved to be more tolerant to feedback delay times. △ Less

Submitted 26 September, 2008; v1 submitted 31 July, 2008; originally announced July 2008.

Journal ref: PhysRevLett.101.220601 (2008)

Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts

Authors: T. P. Martin, A. Szorkovszky, A. P. Micolich, A. R. Hamilton, C. A. Marlow, H. Linke, R. P. Taylor, L. Samuelson

Abstract: The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0… ▽ More The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first subband, six times larger than in GaAs. We report subband spacings in excess of 10 meV, which facilitates quantum transport at higher temperatures. △ Less

Submitted 20 June, 2008; v1 submitted 5 May, 2008; originally announced May 2008.

Comments: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in Applied Physics Letters

Journal ref: Applied Physics Letters 93, 012105 (2008)

Quantum-dot thermometry

Authors: E. A. Hoffmann, N. Nakpathomkun, A. I. Persson, H. A. Nilsson, L. Samuelson, H. Linke

Abstract: We present a method for the measurement of a temperature differential across a single quantum dot that has transmission resonances that are separated in energy by much more than the thermal energy. We determine numerically that the method is accurate to within a few percent across a wide range of parameters. The proposed method measures the temperature of the electrons that enter the quantum dot… ▽ More We present a method for the measurement of a temperature differential across a single quantum dot that has transmission resonances that are separated in energy by much more than the thermal energy. We determine numerically that the method is accurate to within a few percent across a wide range of parameters. The proposed method measures the temperature of the electrons that enter the quantum dot and will be useful in experiments that aim to test theory which predicts quantum dots are highly-efficient thermoelectrics. △ Less

Submitted 11 January, 2008; v1 submitted 5 October, 2007; originally announced October 2007.

Comments: 3 pages, 4 Figures

Journal ref: Appl. Phys. Lett. 91, 252114 (2007)

Effect of time delay on feedback control of a flashing ratchet

Authors: E. M. Craig, B. R. Long, J. M. R. Parrondo, H. Linke

Abstract: It was recently shown that the use of feedback control can improve the performance of a flashing ratchet. We investigate the effect of a time delay in the implementation of feedback control in a closed-loop collective flashing ratchet, using Langevin dynamics simulations. Surprisingly, for a large ensemble, a well-chosen delay time improves the ratchet performance by allowing the system to synch… ▽ More It was recently shown that the use of feedback control can improve the performance of a flashing ratchet. We investigate the effect of a time delay in the implementation of feedback control in a closed-loop collective flashing ratchet, using Langevin dynamics simulations. Surprisingly, for a large ensemble, a well-chosen delay time improves the ratchet performance by allowing the system to synchronize into a quasi-periodic stable mode of oscillation that reproduces the optimal average velocity for a periodically flashing ratchet. For a small ensemble, on the other hand, finite delay times significantly reduce the benefit of feedback control for the time-averaged velocity, because the relevance of information decays on a time scale set by the diffusion time of the particles. Based on these results, we establish that experimental use of feedback control is realistic. △ Less

Submitted 9 November, 2007; v1 submitted 5 July, 2007; originally announced July 2007.

Comments: 6 pages, 6 figures, to appear in Europhysics Letters

Mechanical coupling in flashing ratchets

Authors: Erin M. Craig, Martin J. Zuckermann, Heiner Linke

Abstract: We consider the transport of rigid objects with internal structure in a flashing ratchet potential by investigating the overdamped behavior of a rod-like chain of evenly spaced point particles. In 1D, analytical arguments show that the velocity can reverse direction multiple times in response to changing the size of the chain or the temperature of the heat bath. The physical reason is that the e… ▽ More We consider the transport of rigid objects with internal structure in a flashing ratchet potential by investigating the overdamped behavior of a rod-like chain of evenly spaced point particles. In 1D, analytical arguments show that the velocity can reverse direction multiple times in response to changing the size of the chain or the temperature of the heat bath. The physical reason is that the effective potential experienced by the mechanically coupled objects can have a different symmetry than that of individual objects. All analytical predictions are confirmed by Brownian dynamics simulations. These results may provide a route to simple, coarse-grained models of molecular motor transport that incorporate an object's size and rotational degrees of freedom into the mechanism of transport. △ Less

Submitted 3 February, 2006; originally announced February 2006.

Comments: 9 pages, 10 figures

Concept study for a high-efficiency nanowire-based thermoelectric

Authors: M. F. O'Dwyer, T. E. Humphrey, H. Linke

Abstract: Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunneling structure. The recent advances in growth techniques s… ▽ More Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunneling structure. The recent advances in growth techniques suggest that such devices can now be realized in heterostructured, semiconductor nanowires. Here we propose specific structural parameters for InAs/InP nanowires that may allow the experimental observation of near-Carnot efficient thermoelectric energy conversion in a single nanowire at low temperature. △ Less

Submitted 5 January, 2006; originally announced January 2006.

Self-propelled film-boiling liquids

Authors: H. Linke, B. J. Aleman, L. D. Melling, M. J. Taormina, M. J. Francis, C. C. Dow-Hygelund, V. Narayanan, R. P. Taylor, A. Stout

Abstract: We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchet-like) topology. The pumping effect is observed when the liquid is in the film-boiling regime, for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid. We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchet-like) topology. The pumping effect is observed when the liquid is in the film-boiling regime, for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid. △ Less

Submitted 26 December, 2005; originally announced December 2005.

Nonlinear Effects on Quantum Interference in Electron Billiards

Authors: C. A. Marlow, R. P. Taylor, M. Fairbanks, H. Linke

Abstract: Magnetoconductance fluctuations are used to study the effect of an applied bias on an electron billiard. At lower bias, nonlinear effects can be well described by electron heating alone, while at higher bias (V > 2mV, ~5% of the electron Fermi energy) non-equilibrium effects become significant. At high bias, we also observe that the spectral content of the MCF is sensitive to the nonequilibrium… ▽ More Magnetoconductance fluctuations are used to study the effect of an applied bias on an electron billiard. At lower bias, nonlinear effects can be well described by electron heating alone, while at higher bias (V > 2mV, ~5% of the electron Fermi energy) non-equilibrium effects become significant. At high bias, we also observe that the spectral content of the MCF is sensitive to the nonequilibrium effects. Spectral behavior is consistent with a fractal scaling of the conductance fluctuations with magnetic field, resulting in the first observation of fractal conductance fluctuations outside of the linear regime of transport. △ Less

Submitted 6 December, 2005; originally announced December 2005.

Comments: 4 pages, 2 figures, To be published in a special issue of the Springer Proceedings in Physics Series as part of the proceedings of HCIS-14, Chicago, July 24-29, 2005

Experimental investigation of the breakdown of the Onsager-Casimir relations

Authors: C. A. Marlow, R. P. Taylor, M. Fairbanks, I. Shorubalko, H. Linke

Abstract: We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We experimentally determine that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than one flux quantum) and that its magnitude depends o… ▽ More We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We experimentally determine that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than one flux quantum) and that its magnitude depends on billiard geometry. In addition, we find an unexpected asymmetry in the power spectrum characteristics of the magnetoconductance with respect to reversal of magnetic field and bias voltage. △ Less

Submitted 18 October, 2005; originally announced October 2005.

Comments: ~4 pages, 4 figures, 1 table

Reversible thermoelectric nanomaterials

Authors: T. E. Humphrey, H. Linke

Abstract: Irreversible effects in thermoelectric materials limit their efficiency and economy for applications in power generation and refrigeration. While electron transport is unavoidably irreversible in bulk materials, here we derive conditions under which reversible diffusive electron transport can be achieved in nanostructured thermoelectric materials via the same physical mechanism utilized in quant… ▽ More Irreversible effects in thermoelectric materials limit their efficiency and economy for applications in power generation and refrigeration. While electron transport is unavoidably irreversible in bulk materials, here we derive conditions under which reversible diffusive electron transport can be achieved in nanostructured thermoelectric materials via the same physical mechanism utilized in quantum optical heat engines. Our results may provide a physical explanation for the very high efficiencies recently reported for nanostructured thermoelectric materials such as quantum-dot superlattices. △ Less

Submitted 20 July, 2004; originally announced July 2004.

Comments: 4 pages

Journal ref: Phys. Rev. Lett. Vol. 94 pg. 096601 (2005)

Quantum, cyclic and particle-exchange heat engines

Authors: T. E. Humphrey, H. Linke

Abstract: Differences between the thermodynamic behavior of the three-level amplifier (a quantum heat engine based on a thermally pumped laser) and the classical Carnot cycle are usually attributed to the essentially quantum or discrete nature of the former. Here we provide examples of a number of classical and semiclassical heat engines, such as thermionic, thermoelectric and photovoltaic devices, which… ▽ More Differences between the thermodynamic behavior of the three-level amplifier (a quantum heat engine based on a thermally pumped laser) and the classical Carnot cycle are usually attributed to the essentially quantum or discrete nature of the former. Here we provide examples of a number of classical and semiclassical heat engines, such as thermionic, thermoelectric and photovoltaic devices, which all utilize the same thermodynamic mechanism for achieving reversibility as the three-level amplifier, namely isentropic (but non-isothermal) particle transfer between hot and cold reservoirs. This mechanism is distinct from the isothermal heat transfer required to achieve reversibility in cyclic engines such as the Carnot, Otto or Brayton cycles. We point out that some of the qualitative differences previously uncovered between the three-level amplifier and the Carnot cycle may be attributed to the fact that they are not the same 'type' of heat engine, rather than to the quantum nature of the three-level amplifier per se. △ Less

Submitted 26 November, 2004; v1 submitted 20 July, 2004; originally announced July 2004.

Comments: 9 pages. Proceedings of 'Frontiers of Quantum and Mesoscopic Thermodynamics', Prague 2004

Journal ref: Physica E, Vol. 29 pg 390 (2005)

Inhomogeneously doped thermoelectric nanomaterials

Authors: T. E. Humphrey, H. Linke

Abstract: Inhomogeneously doped thermoelectric nanomaterials with a delta-function electronic density of states can operate with Carnot efficiency in the absence of phonon heat leaks. Here we self-consistently calculate the efficiency and power from open-circuit to short-circuit of a simple model of a thermoelectric nanomaterial with a narrow peak in the electronic density of states and finite lattice the… ▽ More Inhomogeneously doped thermoelectric nanomaterials with a delta-function electronic density of states can operate with Carnot efficiency in the absence of phonon heat leaks. Here we self-consistently calculate the efficiency and power from open-circuit to short-circuit of a simple model of a thermoelectric nanomaterial with a narrow peak in the electronic density of states and finite lattice thermal conductivity, comparing the results for inhomogeneous and homogeneous doping. For power generation between 800K and 300K, we find that not only does inhomogeneous doping increase the maximum efficiency by 10%, but it also increases the maximum power by up to 60%. △ Less

Submitted 20 July, 2004; originally announced July 2004.

Comments: 5 pages. Plenary presentation at the International Thermoelectrics Conference, Adelaide 2004

Optimizing the performance of thermionic devices using energy filtering

Authors: T. E. Humphrey, H. Linke

Abstract: Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons. Here we show that the energy spectrum of electrons transmitted in this way is not optimal, and we derive the ideal energy spectrum for operation in the maximum power regime. By using suitable energy filters, such as resonances in quantum dots,… ▽ More Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons. Here we show that the energy spectrum of electrons transmitted in this way is not optimal, and we derive the ideal energy spectrum for operation in the maximum power regime. By using suitable energy filters, such as resonances in quantum dots, the power of thermionic devices can, in principle, be improved by an order of magnitude. △ Less

Submitted 21 January, 2004; originally announced January 2004.

Comments: 3 pages, 2 figures

Journal ref: J. Phys. D, Vol. 38 pg. 2051 (2005)