Session S15: 2D Materials (Semiconductors) -- Emerging Materials I

An Orbitally-Derived Single Atom Magnetic Memory

Single atoms at the surfaces of solids have demonstrated rich electronic, chemical, and magnetic properties. In this direction, we show that we can manipulate the valency of a single cobalt atom on a crystalline black phosphorus surface. Using the local electric field generated from an STM tip, individual cobalt atoms residing at the same hollow site can be reversibly switched between two stable states, which correspond to the different valencies. Consistency between experimentally observed charge densities and density functional theory calculations reveal distinct high and low total magnetic moments for each state. We investigate the stability of each configuration, as well as compare the experimentally measured impurity states with DFT calculations. Finally, we probe the switching dynamics to determine the underlying mechanism and energy scale of the switching. This system opens up the horizon to explore complex memory based on both the orbital and spin degrees of freedom.   

Anisotropic Quantum Transport in Black Phosphorus Two-dimensional Hole Gas

High-mobility two-dimensional (2D) electron system in black phosphorus van der Waals heterostructures has provided a new platform for studying electron-electron interaction in 2D. In particular, the large anisotropy distinguishes black phosphorus from other high-mobility two-dimensional (2D) electron systems. Here, we observed that new anisotropic states emerge at fractional filling factors for spin-split Landau levels N > 4. The new states manifest as resistance maxima or minima when current flows in different directions. Our results indicate the formation of charge-density-wave stripe phase in black phosphorus 2D hole gas that is surprisingly robust against thermal fluctuation and impurity scattering.   

"Resonant Tunneling in Black Phosphorus Homojunction"

Most common resonant tunnel devices (RTDs) consist of a quantum well formed between double barrier structures. Here for the first time, we demonstrate the RTD in a homojunction. By taking highly anisotropic nature of black-phosphorous (BP) into account, we have fabricated RTDs by sandwiching a thin (4-7 nm)-BP layer between two thick (40-72 nm)-BP flakes. We have observed that RTD could be realized in a BP homojunction by careful alignment of BP flakes. Moreover, a formation of a quantum well at the junction derives from the work function tunability of BP with flake thickness. Work function difference of thick-BP and sandwiched thin-BP layers results in band bending near the junction creating a quantum well. Under certain bias conditions, an electron can tunnel from one side to other through bound states in the quantum well. Such RT has been manifested by observation of negative differential resistance (NDR) in the current-voltage characteristics. The observed NDR peak in the current-voltage characteristics shifts to a higher voltage with decreasing quantum well thickness. Temperature-dependent current-transport reveals that momentum conservation for resonant tunneling is satisfied with acoustic/optical phonon scattering of electrons in the quantum well.   

Strain Tuning of the Electronic Structures of Few-Layer Black Phosphorus

Black phosphorus(BP) is a Van Der Waals material consisted of puckered atomic layers coupled together by the weak Van Der Waals force. Although Van Der Waals force is weak, it plays an important role in their bandstructure evolution. In this work, we tune the bandstructures of few-layer (2-10layer) black phosphorus by applying in-plane biaxial strain. A large tunability has been observed for all of the few-layer BP samples. Careful examination reveals that the strain effect has layer dependence and transition order dependence, which can be accounted by the interlayer coupling effect. Meanwhile, Raman studies for the strained BP have been performed as well, which corroborate our observations.

  

Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump−probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layerdependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron−phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.   

Prolonged photo-carriers generated in Black Phosphorus

Transient electron-hole pairs generated in semiconductors can exhibit unconventional excitonic condensation. Anisotropy in the carrier mass is considered as the key to elongate the life time of the pairs, and hence to stabilize the condensation. By optical pumping, photoexcited carriers facing across the band gap form excitons due to the Coulomb interaction between electrons and holes. As being a direct band gap and highly anisotropic semiconductor, black phosphorus (BP) can be expected to form excitons due to the nonequilibrium electron and hole populations after optical pumping. However, the electrons excited in the conduction band and their carrier dynamics which can offer hints on e-h pair condensation have not yet been clarified.
Here we employ time-resolved ARPES to explore the dynamics of photo-generated carriers in BP. The electronic structure above the Fermi level has been clearly observed, where a massive-and-anisotropic Dirac-type dispersions are confirmed. Importantly, we have directly observe that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps. Such a long duration can be attributed to the stabilized e-h pairs between conduction and valence bands and confirms that the BP is a suitable platform for excitonic condensations.   

Strain-engineered p-type to n-type transition in mono-, bi- and tri-layer phosphorene

Using density functional theory, a detailed computational study is performed to explore the structural and electronic properties of a phosphorene monolayer, bilayer and trilayer under a uniaxial strain along the armchair (baxis) and zigzag (a axis) directions. In the case of a monolayer phosphorene, it is found that strain along the armchair direction slightly affects the a lattice parameter and the puckering height (Δ). Along the zigzag direction, however, variation of the a lattice parameter is compensated by both the a and b lattice parameters while the parameter Δ remains unaffected. In terms of electronic properties, strain along the armchair and zigzag directions changes the nature of the Γ point in the bandgap from a direct to an indirect electronic transition as a function of the strain value. In the strain range from -14% to +6%, all phosphorene structures behave like most semiconductors under strain. However, size and strain combined effect significantly affects the Fermi energy position. Around 0% strain, all phosphorene structures are of p-type, while they switch to an n-type semiconductor in the range of strain values from +2% up to +14%. This p-type to n-type transition may have a major technological impact in fields where mono- and hetero-junctions are needed.   

Probing topology through optical response in group V monolayers

Experiments on Bi monolayers on a SiC substrate reveal an interplay between a huge topologically nontrivial gap ~0.8 eV and strong spin-orbit coupling (SOC), leading to striking transport properties such as a robust quantum spin Hall effect (QSHE)[1]. With a suitable choice of substrates it is also possible to remove valley degeneracy and realize multiple Hall effects in a single materials system [2]. In contrast to transport properties, much less is known about how an optical response could yield topological signatures in these group V monolayers. By combining first-principles calculations for different substrates and a careful inclusion of strong SOC and Coulomb interaction in effective models for these monolayers on various substrates, we show that the changes in optical response reveal topological properties inherent to these systems. We explain how these findings offer new opportunities for proximitized materials [3]. [1] F. Reis, et al., Science 357, 287-290 (2017); F. Dominguez, et al., Phys. Rev. B 98, 161407(R) (2018). [2] T. Zhou, et al., npj Quant. Mater. 3, 39 (2018). [3] I. Zutic, et al., Materials Today (2018).   

Multiple Hall Effects in Functionalized Bismuth Monolayers

Advances in (quantum) spin and anomalous Hall effect, as well as (anomalous) valley Hall effect suggest the electronic degrees of freedom (spin, charge, and valley) can be used as different information carriers [1,2]. The realization of multiple Hall effects in a single 2D material provides a fascinating opportunity to manipulate the implementation of such information [3]. Motivated by the experiments [4], we combine a tight-binding model with first-principles calculations to reveal that with a large nontrivial gap, quantum spin Hall effects (QSHEs) and valley Hall effects appear simultaneously in the functionalized bismuth monolayers [5]. A staggered exchange field is introduced to realize the spin-valley polarized QSHEs. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system [5]. These predicted multiple Hall effects could enable applications of the functionalized bismuth monolayers in electronics, spintronics, and valleytronics.
[1] M. Hasan, et. al., RMP 82, 3045 (2010).
[2] D. Xiao, et. al., RMP 82, 1959 (2010).
[3] T. Zhou, et. al., PRB 94, 235449 (2016).
[4] F. Reis, et al., Science 357, 287 (2017).
[5] T. Zhou, et. al., npj Quant. Mater. 3, 39 (2018).   

Properties and Device Prospects of Emerging Two-Dimensional Materials

In this talk, I will discuss our recent research in studying the electronic, optical and ferroelectric properties of emerging low-dimensional materials, and in developing them for electronic and photonic device applications. The first part of the talk will focus on discussing the basic properties of emerging 2D materials such as black phosphorus and its applications in mid-infrared optoelectronics. Our recent research on the ferroelectric monolayer materials will also be presented. In the second part of the talk, I will discuss our work on applying advanced 4-dimensional imaging technique to study the carrier transport in low dimensional materials and in studying optical phenomenon. I will conclude with remarks on promising future research directions of low-dimensional material properties and devices, and how the emerging materials may benefit future generations of electronics and photonics technology in sensing, imaging and communications.   

Electronic structures and unusually robust bandgap in an ultrahigh-mobility layered oxide semiconductor, Bi2O2Se

Semiconductors are essential materials that affect our everyday life in the modern world. Two-dimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, low-power, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layered air-stable oxide semiconductor, Bi₂O₂Se, with ultrahigh mobility (~2.8 × 10⁵ cm²/V.s at 2.0 K) and moderate bandgap (~0.8 eV). Combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we mapped out the complete band structures of Bi₂O₂Se with key parameters (for example, effective mass, Fermi velocity, and bandgap). The unusual spatial uniformity of the bandgap without undesired in-gap states on the sample surface with up to ~50% defects makes Bi₂O₂Se an ideal semiconductor for future electronic applications. In addition, the structural compatibility between Bi₂O₂Se and interesting perovskite oxides (for example, cuprate high–transition temperature superconductors and commonly used substrate material SrTiO₃) further makes heterostructures between Bi₂O₂Se and these oxides possible platforms for realizing novel physical phenomena.   

Stable holey two-dimensional C2N structures with tunable electronic structure

We investigate the phonon stability of several bulk C₂N holey two-dimensional crystals (C₂N-h2D), a recently synthesized carbon nitride layered material, by using first-principles calculations. Among the polytypes addressed, only one does not display phonon instabilities. The electronic structure evolution of dynamically stable C₂N-h2D from monolayer to bilayer and to bulk is unveiled. The direct band gap at Γ can be decreased by 34% from monolayer to bulk, offering opportunities in optoelectronics. The effective masses of both carriers become smaller as the number of layers increases, and their anisotropy along in-plane directions displayed in the monolayer is reduced, which suggest that the carrier mobility may be tuned as well. These effects are then explained according to the interaction of the orbitals in neighboring layers.   

Chlorine termination of Silicene on Silver (111)

Silicene is the 2D form of silicon, which grows readily on silver (111) as single layers or even bi-layers, showing several surface reconstructions in scanning tunneling microscopy images. Discrete Fourier transform calculations of halide-terminated single and multi-layers predict superconducting as well as topological behavior. In fact, a previous study [1] found possible superconductivity in plain silicene/silver, which – so far – we have been unable to confirm. We successfully grew silicene on silver (111) crystals and imaged them using scanning tunneling microscopy/spectroscopy at 4.2 K. The topographic and spectroscopic results on chlorine-terminated surfaces will be presented.
[1] Lan Chen, et. al., Appl. Phys. Lett. 102, 081602 (2013)