Session E57: 2D Semiconductors: Electronics and Optoelectronics

Live

Electronic exciton polaron is a hypothetical many-body quasiparticle formed by an exciton dressed with a polarized electron-hole cloud in the Fermi sea (FS). It is predicted to display rich many-body physics and unusual roton-like dispersion. The roton-like dispersion has the energy minima at finite momenta for a doped 2D semiconductor because of the Fermi-sea blocking effect. Here, by using reflectance contrast and photoluminescence (PL) spectroscopy, we observe optical signatures of the exciton-polaron Rydberg series and characterize their gate-dependent optical properties in monolayer MoSe₂ and WSe₂. We also established a comprehensive exciton-polaron theory to quantitatively explain our results. Remarkably, the observed exciton polarons exhibit a significant energy gap between the absorption and emission bands, with the gap size increasing from ground to excited state. Such an absorption-emission gap can be attributed to the roton-like exciton-polaron dispersion. Our calculations for the absorption and emission with the roton-effect match very well with the experiments. Such exciton polarons with strong and roton effect shall much enrich the fundamental physics in 2D excitonic systems.   

Live

Field-driven non-volatile change in resistance, also known as memristor effect, has emerged as one of the most important phenomena in the development of components for high-density information storage and brain-inspired or neuromorphic computing. Recently, we discovered memristor effect in atomic monolayers of transitional metal dichalcogenide sandwich structures which has provided a new dimension of interest owing to the prospects of size scaling and the associated benefits. However, the origin of the switching mechanism in atomic sheets remains unclear. Here, using monolayer MoS₂ as a model material, atomistic imaging and spectroscopy reveal that metal substitution into sulfur vacancy results in a non-volatile resistance change. The experimental observations are supported by computational studies of defects and electronic states. These unexpected findings provide an atomistic understanding on the non-volatile switching mechanism and open new avenues for precision defect engineering, down to a singular defect, for realizing the smallest memristor for ultra-dense memory and computing.   

Live

Chemical or electrical doping is an efficient means to improve and control carrier transport and charge injection in materials. Based on a computational-guided experiment, we demonstrate the facile intercalation of chromocene (CrC_P2), an organometallic molecule into the van der Waals (vdW) gap of 2D-based HfS₂. CrC_P2 behaves as pseudo-alkali metals electrostatically doping the host system. The designed HfS₂-CrC_P2 2D-based vdW hybrid system revealed multi-switching capabilities with ultrahigh dynamical control of over 400 folds (i.e., from 1.8 μ/cm to 741 μ/cm) of the cross-planar electrical conductivity. Our findings show a promising route to create an organic/inorganic interface that tunes and tailor the properties of host materials for novel device applications such as quantum and neuromorphic information processing.   

Live

For nanoscale electronic devices, the precise atomic structure of the metal contact significantly affects the performance of the device. We use an accurate DFT-based non-equilibrium Green’s function (NEGF) method with variationally-optimized localized orbitals to study contacts between different metals and graphene and/or graphene nanoribbons (GNRs). For surface metal contacts not chemically bound to graphene, we find that Ti contacts have lower resistance than other metals, such as Au, Ca, Ir, Pt, and Sr. We will discuss channel length effects on the off-state current and the required minimum channel and contact lengths for applications. Depending on the type of the armchair GNR, the transmission gap and current vary dramatically. For an edge contact, zero-bias resistivity with different channel lengths shows diverse patterns compared to that of a surface contact. We also study the oxygen contamination at an edge contact, which lowers the ballistic resistance and introduces a new transmission spectrum.   

Live

The Re atom in Re based transition metal dichalcogenides has a formal d electron count of d³. Considering the 1T and 1H polymorphs that one usually encounters among the layered transition metal dichalcogenides, this should lead to a metallic ground state. However, they are found to be semiconducting with band gaps as large as 1.50 eV and 1.29 eV for ReS₂ and ReSe₂ respectively. Within ab-initio electronic structure calculations using the generalised gradient approximation for the exchange-correlation functional, we are able to reproduce the semiconducting ground state, indicating that electron electron interactions are not responsible for the existence of a band gap. There is also no evidence of magnetic order in these systems. Examining the structure, we find clusters involving primarily four Re atoms connected at their corners. This leads to one-dimensional chains of these clusters running paralllel to each other. A mapping of the electronic structure onto a tight-binding model allows us to selectively examine the role of each structural motif. This is then used to understand the absence of a layer dependence of the electronic structure in contrast to other transition metal dichalcogenides as has been found experimentally [1].


[1] S. Tongay et al., Nat. Comm. 5, 3252 (2014).   

Not Participating

Atomically thin (two-dimensional, 2D) semiconductors have shown great potential as the fundamental building blocks for next-generation electronics. However, all the 2D semiconductors that have been experimentally made so far have room-temperature electron mobility lower than that of bulk silicon, which is not understood. Here, by using first-principles calculations and reformulating the transport equations to isolate and quantify contributions of different mobility-determining factors, we show that the universally low mobility of 2D semiconductors originates from the high “density of scatterings,” which is intrinsic to the 2D material with a parabolic electron band. The density of scatterings characterizes the density of phonons that can interact with the electrons and can be fully determined from the electron and phonon band structures without knowledge of electron-phonon coupling strength. Our work reveals the underlying physics limiting the electron mobility of 2D semiconductors and offers a descriptor to quickly assess the mobility. [1,2,3]
[1] L. Cheng, C. Zhang, Y. Liu, PRL 2020, DOI: 10.1103/PhysRevLett.125.177701 (Editor’s suggestion)
[2] L. Cheng, C. Zhang, Y. Liu, JACS, 2019, DOI: 10.1021/jacs.9b05923
[3] L. Cheng, Y. Liu, JACS, 2018, DOI: 10.1021/jacs.8b07871   

Live

Van der Waals interfaces are unique platforms for novel properties and functionalities. In addition to being building blocks for functionalized devices such as p-n or tunneling junctions, symmetry of the van der Waal interface has recently been attracting much attention, represented by Moiré physics or quasicrystalline structure in twisted interfaces [1-3] and pseudo landau level formation in graphene/black phosphorus heterostructures [4].
In this talk, I will focus on the photocurrent response at van der Waals interfaces. The anomalous photovoltaic effect [6], which indicates the emergence of photo-induced spontaneous current without semiconductor p-n junction nor bias voltage, is observed at the symmetry engineered interface. A potential microscopic mechanism of the anomalous photovoltaic effect will be also discussed.

[1] M. Yankowitz, et al., Nat. Phys. 8, 382 (2012).
[2] C. R. Dean, et al., Nature 497, 598 (2013).
[3] S. J. Ahn et al., Science 361, 782 (2018)
[4] Y. Cao, et al., Nature 556, 43 (2018).
[5] Y. Liu, et al., Nat. Nanotech. 13, (2013) 828.
[6] Y. J. Zhang, et al., Nature 570, 349 (2019).   

Live

In our study, by using micro-focused laser angle-resolved photoemission spectroscopy (ARPES) in combination with the 2D materials manufacturing system (2DMMS) that can freely stack atomic layers by image recognition, machine learning, and autonomous robotic assembly, we developed a high-throughput procedure for investigating the band dispersions of atomically thin micro-flakes. We prepared 2D WTe₂ flake samples for ARPES by using the Graphite / h-BN as a substrate and by encapsulating with graphene. We successfully observed the thickness-dependent band structures of the atomically thin WTe₂ flakes. This sample fabrication procedure for ARPES should be applicable to a wide range of micro-flakes samples, such as heterostructures and twisted materials.   

Live

We study stabilities and electronic properties of hexagonal boron nitride (h-BN) trilayers by using first-principles electronic-structure calculations within the framework of the density functional theory. It has been reported that there are five high-symmetry stacking sequences in the case of the bilayer h-BN, which shows various electronic properties depending on its stacking sequence. It is found that a wider variety of electronic structures can appear in the h-BN trilayer system. It is also found that spatial distributions of wave functions at the valence-band maximum and the conduction-band minimum strongly depend on the stacking sequence of the h-BN trilayer. We further study effects of substitutional doping of a carbon atom on electronic properties of h-BN trilayers. It is found that the layer with the dopant carbon atom can be spatially separated from the conducting layer(s) in several stacking sequences, indicating possibility of the modulation doping in as thin as the trilayer h-BN.   

Live

While the two-dimensional (2D) p-n junctions have been extensively studied for electronic and optoelectronic devices, the semiclassical approaches without considering atomistic details are still insufficient to describe its electronic structures, such as long depletion width and band edge profiles. To overcome such limitations, we combine the multi-space constrained-search density functional theory (MS-DFT) formalism together with the simulation doping method for describing the doped p-n junction under finite-bias conditions. Then, by calculating the lateral WSe₂ p-n junctions, we find that the depletion width calculated within the first-principles approach is several times longer than that of the analytic expressions, which affects the current-voltage characteristics in the 2D p-n junctions. Thanks to the MS-DFT that uniquely allows plotting quasi-Fermi levels (QFLs) profiles within the first-principles calculation, we also extract the QFLs profiles from the lateral WSe₂ p-n junctions under finite-bias conditions. Finally, based on the QFLs profiles and electronic structures, we study the recombination-generation processes of charge carriers inside the depletion layer, including quantum effects.   

On Demand

Gd₂C is one of the famous electrides. We investigate the electronic properties of few-layer Gd₂C that is one of the two-dimensional (2d) electrides. We focus on anionic electrons at the interstitial space. In this study, we employ the Vienna Ab Initio Simulation Package (VASP). The wave functions were expanded using a plane-wave basis set. We presented the electronic properties of the monolayer to the pentalayer of Gd₂C. Moreover, we study the effect of interstitial anionic electrons and surface, compared with bulk state Gd₂C. We also calculate the work functions of the few-layer Gd₂C structures to investigate the layer dependence. The monolayer and the pentalayer have the work functions of 3.426 and 3.350 eV, respectively. Interestingly, the Gd₂C has a lower work function than Y₂C. This tendency of the work function seems to be related to the localization of the electrons in the interstitial space. In other words, the anionic electron density in the interstitial space of Gd₂C is more delocalized than in the case of Y₂C. Finally, we study the response of Gd₂C to the external electric field.