Showing 1–12 of 12 results for author: Chi, D

Reply to: Mobility overestimation in MoS$_2$ transistors due to invasive voltage probes

Authors: Hong Kuan Ng, Du Xiang, Ady Suwardi, Guangwei Hu, Ke Yang, Yunshan Zhao, Tao Liu, Zhonghan Cao, Huajun Liu, Shisheng Li, Jing Cao, Qiang Zhu, Zhaogang Dong, Chee Kiang Ivan Tan, Dongzhi Chi, Cheng-Wei Qiu, Kedar Hippalgaonkar, Goki Eda, Ming Yang, Jing Wu

Abstract: In this reply, we include new experimental results and verify that the observed non-linearity in rippled-MoS$_2$ (leading to mobility kink) is an intrinsic property of a disordered system, rather than contact effects (invasive probes) or other device issues. Noting that Peng Wu's hypothesis is based on a highly ordered ideal system, transfer curves are expected to be linear, and the carrier densit… ▽ More In this reply, we include new experimental results and verify that the observed non-linearity in rippled-MoS$_2$ (leading to mobility kink) is an intrinsic property of a disordered system, rather than contact effects (invasive probes) or other device issues. Noting that Peng Wu's hypothesis is based on a highly ordered ideal system, transfer curves are expected to be linear, and the carrier density is assumed be constant. Wu's model is therefore oversimplified for disordered systems and neglects carrier-density dependent scattering physics. Thus, it is fundamentally incompatible with our rippled-MoS$_2$, and leads to the wrong conclusion. △ Less

Submitted 18 July, 2023; v1 submitted 15 July, 2023; originally announced July 2023.

Momentum Conserved Ultrafast Charge Transfer Dynamics of Interlayer Excitons in vdW Heterostructures

Authors: Pranjal Kumar Gogoi, Yung-Chang Lin, Ryosuke Senga, Hannu-Pekka Komsa, Swee Liang Wong, Dongzhi Chi, Arkady V. Krasheninnikov, Lain-Jong Li, Mark B. H. Breese, Steven J. Pennycook, Andrew T. S. Wee, Kazu Suenaga

Abstract: Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials with unique properties. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. Inevitably,… ▽ More Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials with unique properties. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. Inevitably, the efficiency of 2D heterostructure devices is critically dependent on the charge transfer dynamics. However, the role of the relative rotation angle of the constituent layers on this charge transfer dynamics is hitherto unknown. Here, we investigate MoS$_2$/WSe$_2$ vdW heterostructures (hMWs) using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy (STEM), and report that momentum conservation is a critical factor in the charge transfer dynamics of TMDC vdW heterostructures. The low-loss EEL spectra of the heterostructures with various rotation angles reveal that the charge transfer rate can be about one order-of-magnitude faster in the aligned (or anti-aligned) case than the misaligned cases. These results provide a deeper insight into the role of the fundamental principle of momentum conservation in 2D vdW heterostructure charge transfer dynamics. △ Less

Submitted 1 February, 2019; originally announced February 2019.

Comments: 33 pages, 5 figures

Kondo Impurities in Two Dimensional MoS2 for Achieving Ultrahigh Thermoelectric Powerfactor

Authors: Jing Wu, Yanpeng Liu, Yi Liu, Yongqing Cai, Yunshan Zhao, Hong Kuan Ng, Kenji Watanabe, Takashi Taniguchi, Gang Zhang, Chengwei Qiu, Dongzhi Chi, AH Castro Neto, John TL Thong, Kian Ping Loh, Kedar Hippalgaonkar

Abstract: Local magnetic impurities arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering, and provide a handle for enhancing thermoelectric properties through the Kondo effect. However, the effects of these impurities are often masked by external fluctuations and turbostratic interfaces, therefore, it is highl… ▽ More Local magnetic impurities arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering, and provide a handle for enhancing thermoelectric properties through the Kondo effect. However, the effects of these impurities are often masked by external fluctuations and turbostratic interfaces, therefore, it is highly challenging to probe the correlation between magnetic impurities and thermoelectric parameters experimentally. In this work, we demonstrate that by placing Molybdenum Disulfide on a hexagonal Boron Nitride substrate, a colossal spin splitting of the conduction sub-band up to ~50.0 meV is observed at the sulfur vacancies, suggesting that these are local magnetic states. Transport measurements reveal a large anomalous positive Seebeck coefficient in highly conducting n type MoS2, originating from quasiparticle resonance near the Fermi level described by the Kondo effect. Furthermore, by tuning the chemical potential, a record power factor of 50mW/mK2 in low-dimensional materials was achieved. Our work shows that defect engineering of 2D materials affords a strategy for controlling Kondo impurities and tuning thermoelectric transport. △ Less

Submitted 14 January, 2019; originally announced January 2019.

Comments: 20 pages, 5 figures

Probing physical origin of anisotropic thermal transport in black phosphorus nanoribbons

Authors: Yunshan Zhao, Gang Zhang, Mui Hoon Nai, Guangqian Ding, Dengfeng Li, Yi Liu, Kedar Hippalgaonkar, Chwee Teck Lim, Dongzhi Chi, Baowen Li, Jing Wu, John T L Thong

Abstract: Black phosphorus (BP) has emerged as a promising candidate for next generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behaviour because of its puckered honeycomb structure. Previous studies have demonstrated the thermal transport anisotropy of BP and th… ▽ More Black phosphorus (BP) has emerged as a promising candidate for next generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behaviour because of its puckered honeycomb structure. Previous studies have demonstrated the thermal transport anisotropy of BP and theoretically attribute this to the anisotropy in both phonon dispersion relation and phonon relaxation time. However, the exact origin of such strong anisotropy lacks clarity and has yet to be proven experimentally. In this work, we probe the thermal transport anisotropy of BP nanoribbons (NRs) by an electron beam technique. We provide direct evidence that the origin of this anisotropy is dominated by the anisotropic phonon group velocity for the first time, verified by Young modulus measurements along different directions. It turns out that the ratio of thermal conductivity between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the corresponding Young modulus values. The results from first-principles calculation are consistent with this experimental observation, where anisotropic phonon group velocity between ZZ and AC is shown. Our results provide fundamental insight into the anisotropic thermal transport in low symmetric crystals. △ Less

Submitted 2 September, 2018; originally announced September 2018.

Comments: 14 pages, 5 figures

Chemical Bond-Based Representation of Materials

Authors: Van-Doan Nguyen, Le Dinh Khiet, Pham Tien Lam, Dam Hieu Chi

Abstract: This paper introduces a new representation method that is mainly based on chemical bonds among atoms in materials. Each chemical bond and its surrounded atoms are considered as a unified unit or a local structure that is expected to reflect a part of materials' nature. First, a material is separated into local structures; and then represented as matrices, each of which is computed by using informa… ▽ More This paper introduces a new representation method that is mainly based on chemical bonds among atoms in materials. Each chemical bond and its surrounded atoms are considered as a unified unit or a local structure that is expected to reflect a part of materials' nature. First, a material is separated into local structures; and then represented as matrices, each of which is computed by using information about the corresponding chemical bond as well as orbital-field matrices of two related atoms. After that, all local structures of the material are utilized by using the statistics point of view. In the experiment, the new method was applied into a materials informatics application that aims at predicting atomization energies using QM7 data set. The results of the experiment show that the new method is more effective than two state-of-the-art representation methods in most of the cases. △ Less

Submitted 30 December, 2017; v1 submitted 30 November, 2017; originally announced December 2017.

Nonlinearity in nanomechanical cantilevers

Authors: L. G. Villanueva, R. B. Karabalin, M. H. Matheny, D. Chi, J. E. Sader, M. L. Roukes

Abstract: Euler-Bernoulli beam theory is widely used to successfully predict the linear dynamics of micro- and nano-cantilever beams. However, its capacity to characterize the nonlinear dynamics of these devices has not yet been rigorously assessed, despite its use in nanoelectromechanical systems development. In this article, we report the first highly controlled measurements of the nonlinear response of n… ▽ More Euler-Bernoulli beam theory is widely used to successfully predict the linear dynamics of micro- and nano-cantilever beams. However, its capacity to characterize the nonlinear dynamics of these devices has not yet been rigorously assessed, despite its use in nanoelectromechanical systems development. In this article, we report the first highly controlled measurements of the nonlinear response of nanomechanical cantilevers using an ultra-linear detection system. This is performed for an extensive range of devices to probe the validity of Euler-Bernoulli theory in the nonlinear regime. We find that its predictions deviate strongly from our measurements for the nonlinearity of the fundamental flexural mode, which show a systematic dependence on aspect ratio (length/width) together with random scatter. This contrasts with the second mode, which is always found to be in good agreement with theory. These findings underscore the delicate balance between inertial and geometric nonlinear effects in the fundamental mode, and strongly motivate further work to develop theories beyond the Euler-Bernoulli approximation. △ Less

Submitted 8 January, 2013; originally announced January 2013.

Comments: 24 pages, 6 figures

Grain-boundary grooving and agglomeration of alloy thin films with a slow-diffusing species

Authors: Mathieu Bouville, Dongzhi Chi, David J. Srolovitz

Abstract: We present a general phase-field model for grain-boundary grooving and agglomeration of polycrystalline alloy thin films. In particular, we study the effects of slow-diffusing species on grooving rate. As the groove grows, the slow species becomes concentrated near the groove tip so that further grooving is limited by the rate at which it diffuses away from the tip. At early times the dominant d… ▽ More We present a general phase-field model for grain-boundary grooving and agglomeration of polycrystalline alloy thin films. In particular, we study the effects of slow-diffusing species on grooving rate. As the groove grows, the slow species becomes concentrated near the groove tip so that further grooving is limited by the rate at which it diffuses away from the tip. At early times the dominant diffusion path is along the boundary, while at late times it is parallel to the substrate. This change in path strongly affects the time-dependence of grain boundary grooving and increases the time to agglomeration. The present model provides a tool for agglomeration-resistant thin film alloy design. keywords: phase-field, thermal grooving, diffusion, kinetics, metal silicides △ Less

Submitted 5 November, 2006; originally announced November 2006.

Comments: 4 pages, 6 figures

Journal ref: Phys. Rev. Lett. 98, 085503 (2007)

Interplay between grain boundary grooving, stress, and dealloying in the agglomeration of NiSi1-xGex films

Authors: H. B. Yao, M. Bouville, D. Z. Chi, H. P. Sun, X. Q. Pan, D. J. Srolovitz, D. Mangelinck

Abstract: Germanosilicides, especially those formed on compressive substrates, are less stable than silicides against agglomeration. By studying the solid-state reaction of Ni thin film on strained Si0.8Ge0.2(001), we show that nickel germanosilicide is different from nickel silicide and nickel germanide in several respects: the grains are smaller and faceted, the groove angle is sharper, and dealloying t… ▽ More Germanosilicides, especially those formed on compressive substrates, are less stable than silicides against agglomeration. By studying the solid-state reaction of Ni thin film on strained Si0.8Ge0.2(001), we show that nickel germanosilicide is different from nickel silicide and nickel germanide in several respects: the grains are smaller and faceted, the groove angle is sharper, and dealloying takes place. The germanium out-diffusion creates a stress in the film which favors grooving and agglomeration. △ Less

Submitted 18 May, 2006; originally announced May 2006.

Comments: 13 pages, 4 figures

Journal ref: Electrochem. Solid-State Lett., Volume 10, Issue 2, pp. H53-H55 (2007)

Pyramidal structural defects in erbium silicide thin films

Authors: Eu Jin Tan, Mathieu Bouville, Dong Zhi Chi, Kin Leong Pey, Pooi See Lee, David J. Srolovitz, Chih Hang Tung, Lei Jun Tang

Abstract: A new pyramidal structural defect, 5 to 8 micron wide, has been discovered in thin films of epitaxial erbium disilicide formed by annealing thin Er films on Si(001) substrates at temperatures of 500 to 800C. Since these defects form even upon annealing in vacuum of TiN-capped films their formation is not due to oxidation. The pyramidal defects are absent when the erbium disilicide forms on amorp… ▽ More A new pyramidal structural defect, 5 to 8 micron wide, has been discovered in thin films of epitaxial erbium disilicide formed by annealing thin Er films on Si(001) substrates at temperatures of 500 to 800C. Since these defects form even upon annealing in vacuum of TiN-capped films their formation is not due to oxidation. The pyramidal defects are absent when the erbium disilicide forms on amorphous substrates, which suggests that epitaxial strains play an important role in their formation. We propose that these defects form as a result of the separation of the silicide film from the substrate and its buckling in order to relieve the compressive, biaxial epitaxial stresses. Silicon can then diffuse through the silicide or along the interface to fully or partially fill the void between the buckled erbium disilicide film and the substrate. △ Less

Submitted 10 October, 2005; originally announced October 2005.

Comments: 14 pages, 5 figures. Submitted to Applied Physics Letters

Journal ref: Appl. Phys. Lett. 88, 021908 (2006)

Phase-field model for grain boundary grooving in multi-component thin films

Authors: Mathieu Bouville, Shenyang Hu, Long-Qing Chen, Dongzhi Chi, David J Srolovitz

Abstract: Polycrystalline thin films can be unstable with respect to island formation (agglomeration) through grooving where grain boundaries intersect the free surface and/or thin film-substrate interface. We develop a phase-field model to study the evolution of the phases, composition, microstructure and morphology of such thin films. The phase-field model is quite general, describing compounds and soli… ▽ More Polycrystalline thin films can be unstable with respect to island formation (agglomeration) through grooving where grain boundaries intersect the free surface and/or thin film-substrate interface. We develop a phase-field model to study the evolution of the phases, composition, microstructure and morphology of such thin films. The phase-field model is quite general, describing compounds and solid solution alloys with sufficient freedom to choose solubilities, grain boundary and interface energies, and heats of segregation to all interfaces. We present analytical results which describe the interface profiles, with and without segregation, and confirm them using numerical simulations. We demonstrate that the present model accurately reproduces the theoretical grain boundary groove angles both at and far from equilibrium. As an example, we apply the phase-field model to the special case of a Ni(Pt)Si (Ni/Pt silicide) thin film on an initially flat silicon substrate. △ Less

Submitted 31 August, 2005; originally announced August 2005.

Comments: 12 pages, 5 figures, submitted to Modelling Simulation Mater. Sci. Eng

Journal ref: Modelling Simul. Mater. Sci. Eng. 14, 433 (2006)

Intralayer Carbon Substitution in the MgB2 Superconductor

Authors: T. Takenobu, T. Ito, Dam H. Chi, K. Prassides, Y. Iwasa

Abstract: We report that the ternary MgB2-xCx compounds adopt an isostructural AlB2-type hexagonal structure in a relatively small range of nominal carbon concentration, x<0.1. The lattice parameter a decreases almost linearly with increasing carbon content x, while the c parameter remains unchanged, indicating that carbon is exclusively substituted in the boron honeycomb layer without affecting the inter… ▽ More We report that the ternary MgB2-xCx compounds adopt an isostructural AlB2-type hexagonal structure in a relatively small range of nominal carbon concentration, x<0.1. The lattice parameter a decreases almost linearly with increasing carbon content x, while the c parameter remains unchanged, indicating that carbon is exclusively substituted in the boron honeycomb layer without affecting the interlayer interactions. The superconducting transition temperature Tc, determined by magnetometry experiments, also decreases quasilinearly as a function of the carbon concentration. The structural and electronic behavior of MgB2-xCx displays a remarkable similarity with the isoelectronic Mg1-xAlxB2 despite the different substitution sites. △ Less

Submitted 11 March, 2001; originally announced March 2001.

Comments: 9 pages, 4 figures

Compressibility of the MgB2 Superconductor

Authors: K. Prassides, Y. Iwasa, T. Ito, D. H. Chi, K. Uehara, E. Nishibori, M. Takata, S. Sakata, Y. Ohishi, O. Shimomura, T. Muranaka, J. Akimitsu

Abstract: Considerable excitement has been caused recently by the discovery that the binary boride system with stoichiometry MgB2 is superconducting at the remarkably high temperature of 39 K (1). This potentially opens the way to even higher Tc values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow… ▽ More Considerable excitement has been caused recently by the discovery that the binary boride system with stoichiometry MgB2 is superconducting at the remarkably high temperature of 39 K (1). This potentially opens the way to even higher Tc values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow the understanding of the physics of high-Tc superconductivity without the presence of the multitude of complicated features, associated with the cuprates. Synchrotron X-ray diffraction was used to measure the isothermal compressibility of MgB2, revealing a stiff tightly-packed incompressible solid with only moderate bonding anisotropy between intra- and inter-layer directions. These results, combined with the pressure evolution of the superconducting transition temperature, Tc establish its relation to the B and Mg bonding distances over a broad range of values. △ Less

Submitted 28 February, 2001; originally announced February 2001.

Comments: 6 pages, 3 figures