A.K.M. Newaz

Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

Probing charge scattering mechanisms in suspended graphene by varying its dielectric environment

Graphene with high carrier mobility μ is required both for graphene-based electronic devices and for the investigation of the fundamental properties of Dirac fermions. An attractive approach to increase the mobility is to place graphene in an environment with high static dielectric constant κ that would screen the electric field due to the charged impurities present near graphenes surface. Here we investigate the effect of the dielectric environment of graphene and study electrical transport in multi-terminal graphene devices suspended in liquids with κ ranging from 1.9 to 33. For non-polar liquids (κ<5), we observe a rapid increase of μ(κ), with room-temperature mobility reaching ~60,000 cm(2) Vs(-1) for devices in anisole (κ = 4.3). We associate this trend with dielectric screening of charged impurities adsorbed on graphene. We observe much lower mobility μ~20,000 cm(2) Vs(-1) for devices in polar liquids (κ ≥ 18) and explain it by additional scattering caused by ions present in such liquids.

Electrical Control of near-Field Energy Transfer between Quantum Dots and Two-Dimensional Semiconductors

We investigate near-field energy transfer between chemically synthesized quantum dots (QDs) and two-dimensional semiconductors. We fabricate devices in which electrostatically gated semiconducting monolayer molybdenum disulfide (MoS2) is placed atop a homogeneous self-assembled layer of core-shell CdSSe QDs. We demonstrate efficient nonradiative Förster resonant energy transfer (FRET) from QDs into MoS2 and prove that modest gate-induced variation in the excitonic absorption of MoS2 leads to large (∼500%) changes in the FRET rate. This in turn allows for up to ∼75% electrical modulation of QD photoluminescence intensity. The hybrid QD/MoS2 devices operate within a small voltage range, allow for continuous modification of the QD photoluminescence intensity, and can be used for selective tuning of QDs emitting in the visible-IR range.

Ozone-exposure and annealing effects on graphene-on-SiO2 transistors

We employ resistance measurements and Raman spectroscopy to investigate the effects of UV ozone (UVO) exposure and Ar annealing on graphene-on-SiO2 transistors. Shorter UVO exposures lead to oxygen adsorption and doping; longer exposures lead to significant defect generation and then to etching. Elevated-temperature Ar annealing following UVO exposure leads to local defect healing, as shown by the evolution of the characteristic Raman D- and G-peaks. In striking contrast, the overall graphene transistor resistance increases significantly due to void formation. Density functional calculations show that carbon-oxygen reactions lead to efficient consumption and release of C atoms (as CO or CO2) under conditions of high surface oxygen concentration.

Graphene transistor as a probe for streaming potential.

We explore the dependence of electrical transport in a graphene field effect transistor (GraFET) on the flow of water/sodium chloride electrolyte within the immediate vicinity of that transistor. We find large and reproducible shifts in the charge neutrality point of GraFETs that are dependent on the liquid velocity and the ion concentration. We show that these shifts are consistent with the variation of the local electrochemical potential of the liquid next to graphene that are caused by the fluid flow (streaming potential). Furthermore, we utilize the sensitivity of electrical transport in GraFETs to the parameters of the fluid flow to demonstrate graphene-based mass flow and ionic concentration sensing. We successfully detect a flow as small as ∼70 nL/min and detect a change in the ionic concentration as small as ∼40 nM.

Photoresponse of Natural van der Waals Heterostructures

Van der Waals heterostructures consisting of two-dimensional materials offer a platform to obtain materials by design and are very attractive owing to unique electronic states. Research on 2D van der Waals heterostructures (vdWH) has so far been focused on fabricating individually stacked atomically thin unary or binary crystals. Such systems include graphene, hexagonal boron nitride, and members of the transition metal dichalcogenide family. Here we present our experimental study of the optoelectronic properties of a naturally occurring vdWH, known as franckeite, which is a complex layered crystal composed of lead, tin, antimony, iron, and sulfur. We present here that thin film franckeite (60 nm < d < 100 nm) behaves as a narrow band gap semiconductor demonstrating a wide-band photoresponse. We have observed the band-edge transition at ∼1500 nm (∼830 meV) and high external quantum efficiency (EQE ≈ 3%) at room temperature. Laser-power-resolved and temperature-resolved photocurrent measurements reveal that the photocarrier generation and recombination are dominated by continuously distributed trap states within the band gap. To understand wavelength-resolved photocurrent, we also calculated the optical absorption properties via density functional theory. Finally, we have shown that the device has a fast photoresponse with a rise time as fast as ∼1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH, and may pave an avenue toward developing nanoscale optoelectronic devices with tailored properties.

Electrical Stress and Total Ionizing Dose Effects on

Electrical stress and 10-keV x-ray irradiation and annealing responses are evaluated for back-gate MoS2 transistors. Relative stability of device characteristics is observed for constant-voltage stress. The drain current decreases significantly after both positive and negative bias irradiation. Density functional theory calculations and ozone exposure experiments suggest that O atoms adsorbed on the MoS2 surface during 10-keV x-ray irradiation function as electron traps, causing mobility degradation and voltage shifts.