Ivan Verzhbitskiy

Bottom-Up Synthesis of Liquid-Phase-Processable Graphene Nanoribbons with Near-Infrared Absorption

Structurally defined, long (>100 nm), and low-band-gap (∼1.2 eV) graphene nanoribbons (GNRs) were synthesized through a bottom-up approach, enabling GNRs with a broad absorption spanning into the near-infrared (NIR) region. The chemical identity of GNRs was validated by IR, Raman, solid-state NMR, and UV-vis-NIR absorption spectroscopy. Atomic force microscopy revealed well-ordered self-assembled monolayers of uniform GNRs on a graphite surface upon deposition from the liquid phase. The broad absorption of the low-band-gap GNRs enables their detailed characterization by Raman and time-resolved terahertz photoconductivity spectroscopy with excitation at multiple wavelengths, including the NIR region, which provides further insights into the fundamental physical properties of such graphene nanostructures.

Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus

Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity ∼3 × 10(13) Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to ∼9 × 10(4) A W(-1) by applying a source-drain bias (VSD) of 3 V, which is the highest ever measured in any 2D material and 10(7) times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature.

Evidence for Fast Interlayer Energy Transfer in MoSe2/WS2 Heterostructures

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering.

Exciton–Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles

Hybrid systems of excitons strongly coupled to localized surface plasmons supported by metallic nanoparticles define a new approach to control light-matter interactions. Here, we report exciton-plasmon coupling in two-dimensional (2D) semiconductors, such as MoS2 and WS2, hybridized with silver nanoparticles. Prominent photoluminescence enhancement in monolayer MoS2 was observed with localized surface plasmon resonance (LSPR) tuned to the exciton resonance. By tuning the excitation energy, the contributions from near field enhancement and radiative emission rate enhancement via Purcell effect were resolved. Strong coherent dipole-dipole coupling between excitons and LSPR in resonant condition manifests as an electromagnetically induced transparency window in the extinction spectra of the localized surface plasmon. In this strong coupling regime a new quasi-particle, known as a plexciton, is expected to exhibit distinct properties, which exist in neither of the original particles. Our results demonstrate that 2D semiconductors hybridized with plasmonic structures not only hold great promise in the applications of energy-harvesting and light-emitting devices, but also provide an attractive platform for fundamental investigations of exciton-plasmon interactions in the strong coupling regime.Exciton-plasmon coupling in hybrids of a monolayer transition metal dichalcogenide and Ag nanoparticles is investigated in the weak and strong coupling regimes. In the weak coupling regime, both absorption enhancement and the Purcell effect collectively modify the photoluminescence properties of the semiconductor. In the strong coupling regime, electromagnetically induced transparency dips are displayed, evidencing coherent energy exchange between excitons and plasmons.

Engineering Bandgaps of Monolayer MoS2 and WS2 on Fluoropolymer Substrates by Electrostatically Tuned Many-Body Effects.

Intrinsic electrical and excitonic properties of monolayer transition-metal dichalcogenides can be revealed on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid-state back gates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandgap renormalization, Pauli blocking, and carrier screening against carrier doping.

Raman Fingerprints of Atomically Precise Graphene Nanoribbons.

Bottom-up approaches allow the production of ultranarrow and atomically precise graphene nanoribbons (GNRs) with electronic and optical properties controlled by the specific atomic structure. Combining Raman spectroscopy and ab initio simulations, we show that GNR width, edge geometry, and functional groups all influence their Raman spectra. The low-energy spectral region below 1000 cm–1 is particularly sensitive to edge morphology and functionalization, while the D peak dispersion can be used to uniquely fingerprint the presence of GNRs and differentiates them from other sp2 carbon nanostructures.

Tunable D peak in gated graphene

AbstractWe report the gate-modulated Raman spectrum of defective graphene. We show that the intensity of the D peak can be reversibly tuned by applying a gate voltage. This effect is attributed to chemical functionalization of the graphene crystal lattice, generated by an electrochemical reaction involving the water layer trapped at the interface between silicon and graphene.

Efficient Carrier-to-Exciton Conversion in Field Emission Tunnel Diodes Based on MIS-Type van der Waals Heterostack

We report on efficient carrier-to-exciton conversion and planar electroluminescence from tunnel diodes based on a metal-insulator-semiconductor (MIS) van der Waals heterostack consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS2). These devices exhibit excitonic electroluminescence with extremely low threshold current density of a few pA·μm-2, which is several orders of magnitude lower compared to the previously reported values for the best planar EL devices. Using a reference dye, we estimate the EL quantum efficiency to be ∼1% at low current density limit, which is of the same order of magnitude as photoluminescence quantum yield at the equivalent excitation rate. Our observations reveal that the efficiency of our devices is not limited by carrier-to-exciton conversion efficiency but by the inherent exciton-to-photon yield of the material. The device characteristics indicate that the light emission is triggered by injection of hot minority carriers (holes) to n-doped WS2 by Fowler-Nordheim tunneling and that hBN serves as an efficient hole-transport and electron-blocking layer. Our findings offer insight into the intelligent design of van der Waals heterostructures and avenues for realizing efficient excitonic devices.

Strong Optical Absorption and Photocarrier Relaxation in 2-D Semiconductors

Strongly bound excitons in the 2-D crystals of transition metal dichalcogenides give rise to characteristic optical absorption and emission features. Here, we review and provide additional experimental evidence on the unusually strong optical absorption features of monolayer molybdenum disulfide (MoS2) arising from band nesting and corresponding divergence of joint density of states, which is a distinct signature of 2-D systems. We study low-temperature photoluminescence excitation spectra of suspended MoS4 and discuss spontaneous formation of indirect excitons for excitation at the band nesting point.

Chalcogenide Nanosheets: Optical Signatures of Many-Body Effects and Electronic Band Structure

Layered chalcogenide materials exhibit a wide range of physical properties associated with their quasi-2D nature and have been studied extensively since the second half of the twentieth century. Following the discovery of graphene, the ability to isolate individual monolayer of these material has recently opened up numerous avenues for probing various physical effects in the ultimate 2D confinement limit. Monolayers of group 6 transition metal dichalcogenides are an attractive platform for studying many-body effects, non-linear optics, and valley physics. Along with other emerging 2D chalcogenides, they offer unique opportunities for realizing novel devices and their technological implementations. Here we review the fundamental properties of various semiconducting chalcogenide nanosheets and their heterostructures with emphasis on the electronic structure and optical properties of Mo- and W-based dichalcogenides (MoS2, MoSe2, MoTe2, WS2, WSe2). We discuss the current understanding on the the layer-dependent energy dispersion and and its optical signatures in these material systems. We further discuss the strong excitonic effects and review recent experimental efforts in estimating exciton binding energy using various optical and opto-electrical approaches.