Session V13: 2D Materials -- Optical Properties

Toward Quantum Optoelectronics and Optomechanics in Flatland

Transition metal dichalcogenide monolayers and multiayers are atomically thin semiconductors that support tightly bound intra- and inter-layer excitons. In this presentation, I will describe our recent efforts to realize solid-state quantum optoelectronic devices using these atomically thin materials. In particular, I will describe how we prepare high-quality van der Waals heterostructures that feature spatially homogeneous, nearly lifetime-broadened excitons. I will then discuss the spectroscopy of intra- and inter-layer excitons in these structures and how we use them to realize atomically thin, electrically tunable mirrors. I will also describe a new approach for dynamically manipulating the exciton dynamics in these structures via electromechanical control over the suspended structures. I will end the presentation by discussing the ongoing efforts in my group to realize new quantum optical/optoelectronic/optomechanical effects in these structures.
  

THz Electrical Response of Graphene Antennas

Studies of high frequency electrical properties offer an important route in exploring the under explored unique characteristics of graphene. Here the GHz to THz response of graphene is investigated as a proof of principle concept for use as an ultrawide band patch antenna as part of a graphene based near-field communication system. We show that it is possible to produce a graphene multimode variable surface impedance microstrip antenna with several hundred GHz bandwidth via careful selection of the antenna geometry and substrate, and via engineering the relative contributions of the intra- and interband transitions. For an operating frequency of 2 THz an optimized return loss of -26 dB, bandwidth of 504 GHz and an antenna efficiency of -3.4 dB are calculated. Arising from the circular nature of the antenna structure higher order modes are possible and at 3.5 THz the antenna efficiency improves to -0.36 dB however this is accompanied by a reduction in the bandwidth to 200 GHz. Calculations of the real and imaginary parts of the dielectric constant between 1 and 10 THz have been performed which may be important for metamaterial applications.
  

Attosecond Dynamics and Electronic Rehybridization in MoS2

Two-dimensional transition metal dichalcogenides exhibit highly unusual and layer-dependent electronic structure, caused by quantum confinement effects. The results of these changes have been widely documented in their valley-dependent optical properties. The impact of electron-electron interactions on the carrier dynamics is however at present less clear. Here we report on the attosecond dynamics in MoS₂ as a function of carrier density in the conduction band. We show that increased electron density in the conduction band leads to orbital rehybridization, which is the source of the observed layer decoupling. These effects are also clearly observed in the electronic structure in the conduction band region. Our results uncover the many-body effects that govern the electronic properties of MoS₂ and likely other 2D materials.   

Quantum optics with atomically thin materials

Two-dimensional, atomically-thin, materials have received enormous interest as a result of their unique mechanical, electrical and optical properties. Although these materials have been investigated for applications in opto-electronics, not much work has focused on these systems as a platform for quantum photonics and quantum optics. In this talk I will describe some approaches that leverage atomically thin semiconductors, and other two-dimensional materials, assembled in layered van der Waals heterostructures for applications in these areas. In the first part of the talk I will describe the unique photophysical properties of quantum emitters hosted by single layer two-dimensional materials. I will describe our recent efforts to controllably charge the quantum emitters and realize a localized spin-valley-photon interface. I will also present results on valley-polaritons that are a manifestation of many body physics arising when coupling an atomically thin semiconductor to a planar optical cavity.
  

Tunable nonlinearity in a Graphene-Silicon Nitride hybrid resonator

Silicon Nitride (SiNx) resonators with high Q have garnered attention due to their strong promise of operation in the quantum regime at room temperature. Simultaneously, graphene resonators with high Young’s modulus leading to large and tunable mechanical nonlinearity, have opened up possibilities of mixing and manipulating frequency modes. Here we propose a hybrid system that integrates high Q of SiNx resonator with nonlinearity of graphene resonator. When both resonator modes are tuned on resonance, the motional backaction of graphene induces nonlinear response in SiNx. The hybrid mode, when driven parametrically at twice the resonant frequency, demonstrates parametric amplification and cascaded four wave mixing. Generated peaks and their dispersion reveal the rich interplay between nonlinear damping and cubic nonlinearity. Observations match well with numerical simulations. Furthermore, in direct analogy with optomechanics, the graphene acts as an auxiliary cavity leading to induced nonlinearity on SiNx at resonance. These results indicate that such a hybrid mechanical system can be an efficient, alternate platform for coherent control of modes, in the growing field of opto and electromechanics.
  

Ambient effects on hysteresis and photogating in MoS2 photodetectors

Transition metal dichalcogenides (TMDs) are ideal candidates to create ultra-thin optoelectronics that can be flexible and semitransparent. Photodetectors based on TMDs have demonstrated remarkable performance, with high responsivity and high detectivity ^[1], however these devices are hindered by a slow response time caused by charge trapping. Gas adsorbents which create charge traps, are a main contribution to hysteresis and photogating ^[2,3]. Here we study the effect of ambient conditions on the performance of MoS₂ photodetectors through vacuum pumping and illumination.

Reference:
1. Han, P., Marie, L.S., Wang, Q.X., Quirk, N., El Fatimy, A., Ishigami, M. and Barbara, P., 2018. Highly sensitive MoS2 photodetectors with graphene contacts. Nanotechnology, 29(20), p.20LT01.

2. Ahn, J.H., Parkin, W.M., Naylor, C.H., Johnson, A.C. and Drndić, M., 2017. Ambient effects on electrical characteristics of CVD-grown monolayer MoS 2 field-effect transistors. Scientific Reports, 7(1), p.4075.

3. Di Bartolomeo, A., Genovese, L., Giubileo, F., Iemmo, L., Luongo, G., Foller, T. and Schleberger, M., 2017. Hysteresis in the transfer characteristics of MoS2 transistors. 2D Materials, 5(1), p.015014.   

Temperature Dependence of the Photoconduction in the Few-Layered ReSe2 Field-Effect Transistor

We will present the photo response of the few-layered ReSe₂ field-effect transistor fabricated using mechanically exfoliated crystals grown using the chemical vapor transport method. The temperature dependence of the photoconductivity was measured as a function of temperature from 20 K to 300 K using a continuous laser source (λ = 640 nm; E = 1.94eV), over a broad range of illuminating laser power, P_eff (0.2 nW < P_eff < 84 nW). We measured the power dependence of the steady state photocurrent (I_ph) on P_eff (I_ph ~ (P_eff)^γ, where 0.3 ≤ γ ≤ 0.8). The highest responsivity (R) and external quantum efficiency (EQE) obtained from the few-layered ReSe₂ phototransistors at P_eff = 0.2 nW were ~ 2150 A/W and 10⁵ %, respectively. These findings and the temperature dependence of the photoconductivity as a function of gate voltage will be presented.   

Temperature dependent electrical and photoconductive properties of few-layer Indium Selenide (InSe) FETs

Here we report on the temperature dependent electrical and photoconductive properties of n-type few-layer InSe Field Effect Transistors (FETs), micromechanically exfoliated from bulk crystal grown using chemical vapor transport method. The temperature dependent electrical conduction in these InSe FETs shows two distinct conduction mechanisms. At high temperatures electrical conduction is governed by thermally activated behavior, while at low temperatures electrical conduction follows Mott’s 2D variable range hopping mechanism. Further, photoconductivity measurements show that these InSe FETs display room temperature photo-responsivities of ~ 0.05 AW^-1 when illuminated with a laser of wavelength λ = 658 nm and a power of ~23 nW. The photo-responsivities of these FETs showed several orders of magnitude improvement (up to ~ 15 AW^-1) upon application of a gate voltage (V_G = 60 V). These key findings and temperature dependent photoconductivity study will be presented and discussed.   

Electric field effect studies of atomically thin crystals of ferroelectric In2Se3

Ultrathin films of ferroelectric materials are important for applications, including non-volatile memories, electromechanical actuators, and sensors. For a ferroelectric film of transition metal oxide to possess electrically switchable polarization, a minimal film thickness is required. In₂Se₃ is a layered III–VI compound with the unit cell consisting of a quintuple layer (QL) of In and Se ions in a triangular lattice featuring covalent bonding within the unit cell but van der Waals bonding between neighboring unit cells. It was predicted recently that In₂Se₃ is ferroelectric down to single unit-cell thickness, possessing both in- and out-of-plane polarizations in the a- and b-phases of In₂Se₃ polymorphism, respectively. We have carried out electric field effect studies of atomically thin crystals of a-In₂Se₃ down to liquid helium temperatures. These 2D crystals were prepared by mechanical exfoliation and characterized by Raman spectroscopy, photoluminescence, and second harmonic generation measurements. We found that the crystals are easily gated to possess a finite electrical conductivity characterized by thermally activated behavior. Source-drain current vs. gate voltage measurements revealed a hysteresis loop in the transfer characteristic, typical for ferroelectric devices.   

Crossover between Photochemical and Photothermal Oxidations of Atomically Thin CrPS4

Two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have optical bandgaps that can be tuned via thickness, strain, dielectric screening and intercalation, and thus stand promising candidates for optoelectronic devices. Despite the potential, however, many including phosphorene and CrI₃ lack photostability even in the ambient air. Here, we studied photoreaction mechanisms of 2D chromium thiophosphate (CrPS₄), an antiferromagnetic semiconductor, using Raman spectroscopy and atomic force microscopy (AFM). Few-layer CrPS₄ underwent photodegradation which accelerated with increasing partial pressure of O₂. Topographic analysis by AFM showed that 2D CrPS₄ is photo-oxidized at a laser power density two orders of magnitude lower than that required for MoS₂ obeying a photothermal mechanism. Based on various control experiments over photon energy, power density and humidity, we propose that the low-power photoreaction of CrPS₄ is a one-photon photochemical oxidation mediated by CrPS₄-sensitized generation of singlet O₂ and is switched over to a photothermal oxidation in a high-power regime. Additionally, the efficacy of thin Al₂O₃ films will be discussed as protecting layers of 2D CrPS₄.
  

Spectroscopic Characterization of Interlayer Contaminants within van der Waals Heterostructures

Van der Waals heterostructures (vdWHs) leverage the characteristics of disparate 2D material building blocks to create a myriad of structures with unique and desirable properties. A common fabrication strategy relies on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of structures produced in this way frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers. This contamination impedes the study and application of intrinsic heterostructures requiring pristine interfaces. Here, we use a photothermal induced resonance technique to obtain nanoscale infrared spectra and maps of the contamination with ~20 nm spatial resolution. Heterostructures comprised of WS₂, WSe₂, and hBN layers were found to contain significant amounts of polydimethylsiloxane and polycarbonate, corresponding to the stamp materials used in their construction. Additionally, we validate spectroscopically a previously reported “nano-squeegee” technique as an effective means of locally removing contaminants. These insights into the chemical makeup and sources corrupting vdWHs provide guidance for devising mitigation strategies and enhance capabilities for producing materials with precisely engineered properties.