Session Z17: 2D Crystals: Beyond Graphene

Electronic and Transport Properties of Few-Layer MoS$_{2}$ Crystals

We investigate the electronic properties of few-layer MoS$_{2}$ flakes prepared by mechanical exfoliation. Field-effect transistors from MoS$_{2}$ flakes were fabricated and their properties were systematically characterized as a function of sample thickness. Scanning probe measurements are employed to characterize the interface between MoS$_{2}$ flakes and metal contacts. Transport properties of these devices and their correlation to electronic structure calculations are discussed.   

Optical pump-probe studies of carrier dynamics in few-layer MoS$_2$

Molybdenum disulfide is a transition metal dichalcogenide with an indirect band gap of 1.29 eV. Its layered structure allows fabrication of atomically-thin films where the quantum confinement can significantly modify the electronic and optical properties. We demonstrate that a femtosecond pump-probe technique can be used to study charge carrier dynamics in few-layer MoS$_2$ samples fabricated on Si/SiO$_2$ substrates by mechanical exfoliation. Carriers are injected by a 780-nm pump pulse via phonon assisted indirect absorption in bilayers or through edge states. Their dynamics are probed by measuring differential reflection of a probe pulse tuned to the excitonic transition near 670 nm. We found that the magnitude, the sign, and the decay time of the signal change dramatically as the probe wavelength is tuned within the excitonic resonance. With a fixed probe wavelength, the differential reflection signal is proportional to the carrier density. The decay time is independent of the carrier density. Besides providing quantitative information on the carrier dynamics in this promising two-dimensional material, our experiment may stimulate further optical studies of carrier dynamics in this material system.   

Integrated circuits and logic operations with high room temperature voltage gain based on single-layer MoS$_{2}$

Two-dimensional materials such as single-layer MoS2 represent the ultimate limit of miniaturization in the vertical dimension, are interesting as building blocks of low-power nanoelectronic devices and are suitable for integration due to their planar geometry. Because they are less than 1 nm thin, 2D materials in transistors could also lead to reduced short channel effects and result in fabrication of smaller and more power efficient transistors. Here, we report on the first integrated circuit based on a two-dimensional semiconductor MoS2. Our integrated circuits are capable of operating as inverters, converting logical ``1'' into logical ``0'', with room-temperature voltage gain higher than 4.5, making them suitable for incorporation into digital circuits. We also show that electrical circuits composed of single-layer MoS2 transistors are capable of performing the NOR logic operation, the basis from which all logical operations and full digital functionality can be deduced. We have also fabricated suspended single-layer MoS2 membranes and have performed mechanical measurements using an atomic force microscope. Our results show that single-layer MoS2 has a Young's modulus higher than steel and can withstand deformation up to 11{\%}, making it suitable for integration with flexible electronic devices.   

Transient Photoluminescence in MoS$_{2}$ layered crystals

We report sub-10-ps transient exciton photoluminescence (PL) in mechanically exfoliated few- and mono-layered crystals of MoS$_{2}$. We characterize layered crystals with thickness of $\sim $1$\mu $m, 100 nm, 10nm, and down to few-layers on SiO2/Si and mica substrates using luminescence and Raman spectroscopy spectroscopy. A frequency shift of $\sim $2 cm$^{-1}$ is observed on sub-10-nm-thick samples for the in-plane $E_{2g}^1 $ and the out-of-plane $A_{1g} $ Raman modes. The relative intensities of Stokes and Anti-Stokes Raman components are used to determine the lattice temperature under a laser excitation with a spot diameter of 1 $\mu $m and an average power 0.5 to 20mW. PL spectra are measured for lattice temperatures from $\sim $70K to 500K. We observe two groups of luminescence emissions with comparable peak intensities centered at 1.85eV (VIS) and 1.35eV for samples of a thickness 1 $\mu $m down to 10 nm under a cw laser excitation at a wavelength of 532nm (2.33eV). The VIS luminescence emissions are enhanced under a 2-ps pulsed laser excitation at a wavelength of 633nm (1.96eV). The rise and decay times of the luminescence are found to be less than 5 ps. Our results suggest that excitonic effects play a role in enhancing the luminescence quantum efficiency.   

Quasiparticle self-consistent GW calculations of monolayer, bilayer and bulk MoS$_{2}$

Photoluminescence and absorption spectra of monolayer MoS$_{2}$ indicate a direct gap behavior while bulk MoS$_{2}$ is known to have an indirect gap. The details of these spectra and the band structure are not yet fully understood. Here, the quasiparticle self-consistent GW method is used to study the electronic structure of monolayer, bilayer, and bulk MoS$_{2}$. Band structures, effective masses, and dielectric functions are extracted from our calculations. In contrast to another recent GW calculation, we find the monolayer to have a direct gap of 2.84 eV at K, which is large compared to the photoluminescence energies. The exciton binding energy for this transition is estimated within an effective mass approximation using our calculated effective masses and dielectric constants and amounts to about 0.90 eV, leading to good agreement with the experimental data for the lowest direct transition. We find a second conduction band local minimum along $\Gamma $-K about 0.44 eV higher but do not find it to give rise to a bound state exciton. When spin-orbit coupling is included, we find a spin-splitting of the levels along $\Gamma $-K in the monolayer related to the absence of an inversion center. In the bilayer, we find an indirect gap from $\Gamma $-K and a splitting of the valence band at K, mainly due to the interlayer interaction but also increased by spin-orbit coupling. The splitting between the lowest two absorption features A and B is consistent with the slightly larger splitting calculated in the bilayer than the monolayer.   

High Performance Two Dimensional MoS$_{2}$ MOSFET with ALD Al$_{2}$O$_{3}$ Gate Stacks

We study the growth mechanism of atomic layer deposition (ALD) of Al$_{2}$O$_{3}$ on layered MoS$_{2}$. We demonstrate the feasibility of direct growth of Al$_{2}$O$_{3}$ on this 2D material by trimethylaluminum (TMA) and water as precursors. Atomic force microscopy study shows that the quality of the Al$_{2}$O$_{3}$ film is degraded at elevated temperatures, originated from impeded surface absorption of precursors. We also apply density functional theory (DFT) study of the reaction which is in good agreement with our experimental observations. In addition, we fabricate dual gate MoS$_{2}$ metal-oxide-semiconductor field effect transistors (MOSFET). From the transport study we find out the lowering the growth temperature will result in a huge negative threshold voltage shift, which can be improved by either forming gas anneal after Al$_{2}$O$_{3}$ deposition or insertion of an Al seeding layer which would facilitate higher growth temperature with better film quality. Further details will be provided in the presentation.   

Novel valley and spin physics in monolayer MoS$_{2}$

We show that the valley Hall effect and valley-dependent optical selection rule can be realized in monolayer MoS$_{2}$, which is a direct bandgap semiconductor with non-central valleys. In addition, spin-orbit coupling in this materials is large, which gives rise to novel spin physics tied to the valley degree of freedom.   

Enhanced thermal conductivity and isotope effect in single-layer hexagonal boron nitride

We have calculated the lattice thermal conductivity, $k$, of both naturally occurring and isotopically enriched single layers of hexagonal boron nitride (h-BN) as well as bulk h-BN using an exact numerical solution of the Boltzmann transport equation for phonons [1]. Good agreement is obtained with measured bulk h-BN data [2], and the stronger phonon-phonon scattering identified in these systems explains why their $k$ values are significantly lower than those in graphene and graphite. A reduction in such scattering in the single layer arising mainly from a symmetry-based selection rule leads to a substantial increase in$ k$, with calculated room temperature values of more than 600 W/m-K. Additional enhancement is obtained from isotopic enrichment, which exhibits a strong peak as a function of temperature, with magnitude growing rapidly with crystallite size. [1] L. Lindsay and D. A. Broido, Phys. Rev. B 84, 155421 (2011). [2] E. K. Sichel, R. E. Miller, M. S. Abrahams, and C. J. Buiocchi, Phys. Rev. B 13, 4607 (1976).   

Two-dimensional Dirac fermions and quantum transport in (Sr/Ca)MnBi$_{2}$

We report two dimensional Dirac fermions and quantum transport behavior in single crystals of SrMnBi$_{2}$ and CaMnBi$_{2}$. The non-zero Berry's phase, small cyclotron resonant mass and first-principle band structure suggest the existence of the Dirac fermions in the Bi square nets. Angular dependent magnetoresistance and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^{\ast }$ from semiclassical weak-field $B^{2}$ dependence to the high-field unsaturated linear magnetoresistance ($\sim $120{\%} in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The temperature dependence of $B^{\ast }$ satisfies quadratic behavior, which is attributed to the splitting of linear energy dispersion in high field. Our results demonstrate the universal existence of two dimensional Dirac fermions in different materials with Bi square nets.   

Field-induced polarization of Dirac valleys in bismuth

The principal challenge in the field of ``valleytronics'' is to lift the valley degeneracy of electrons in a controlled way. In graphene, a number of methods to generate a valley-polarized flow of electrons have been proposed, which are yet to be experimentally realized. In bulk semi-metallic bismuth, the Fermi surface includes three cigar-shaped electron valleys lying almost perpendicular to the high-symmetry axis known as the trigonal axis. The in-plane mass anisotropy of each valley exceeds 200 as a consequence of Dirac dispersion, which drastically reduces the effective mass along two out of the three orientations. We present a study of angle-dependent magnetoresistance in bismuth which shows that a flow of Dirac electrons along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. The effect is visible even at room temperature. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. At high temperature and low magnetic field, the three valleys are interchangeable and the three-fold symmetry of the underlying lattice is respected. As the temperature is decreased or the magnetic field increased, this symmetry is spontaneously lost. This loss may be an experimental manifestation of the recently proposed valley-nematic Fermi liquid state.   

STM measurements of Charge Density Waves in S-doped NbSe$_{2}$

Recent scanning tunneling microscopy (STM) measurements have shown that crystal defects have a profound influence on the nature of the charge density wave (CDW) transition in the transition-metal dichalcogenides (TMD). In order to examine this effect systematically, we employ low temperature STM to visualize the CDW transition in intentionally doped TMD crystals. We use a newly designed homebuilt ultra-low-loss, variable temperature STM to perform measurements on crystal samples of NbS$_{x}$Se$_{2-x}$. We describe the effect of the sulphur atoms on both the local and global charge order in this material.   

Imaging Charge Density Wave Nucleation in NbSe$_{2}$

Understanding the effects of spatial inhomogeneity in complex materials is necessary to achieve a fundamental understanding of their quantum states. NbSe$_{2}$ serves as a clean and relatively simple system for understanding the emergence of one such state -- the charge density wave (CDW) phase. Using variable temperature scanning tunneling microscopy (STM), we visualize the nucleation of CDWs about crystal defects at temperatures well above T$_{CDW}$. The CDW correlation length increases with decreasing temperature, until global order is reached below T$_{CDW}$. We also employ scanning tunneling spectroscopy in order to visualize the energy-dependent, spatial phase of the CDW state. With both topographic and spectroscopic data, we will provide a clear picture of the CDW transition and insight into the microscopic mechanisms at work.   

Nature of Electronic States in Ultrathin MoS$_{2}$ Field Effect Transistor

Molybdenum disulphide (MoS$_{2})$ is a layered transition metal dichalcogenide with a Mo layer sandwiched between two S layers (S-Mo-S), which forms its basic unit. Each basic unit is attached to other units only with weak Van der Waals force. This enables to make an atomically thin single layer of MoS$_{2}$ with a bandgap 1.9 eV. The presence of bandgap has made it an interesting material in thin film transistors. It has been reported [1] recently that very high on/off ratio ($\sim $10$^{8})$ can be obtained in single layer MoS$_{2}$ transistor due to the presence of this bandgap. Though the on/off ration is very high, mobility in these transistors are considerably low. Here we have investigated the origin of such low mobility. From our temperature dependent study we find that atomically thin MoS$_{2}$ layer becomes highly disordered in the presence of the substrate and electron got localised in the traps created by the charge impurities at substrate-MoS$_{2}$ interface. We propose that high mobility can be obtained in these transistors by removing the charge impurity background. \\[4pt] [1] Radisavljevic, B. \textit{et al}. Nature Nanotechnology \textbf{2011}, 6, 147--150. \\[0pt] [2] Ghatak, S. \textit{et al}. ACS Nano \textbf{2011}, 5, 7707.