Hybrid graphene photoconductor/phototransistor has achieved giant photoresponsivity, but its response speed dramatically degrades as the expense due to the long lifetime of trapped interfacial carriers. In this work, by intercalating a large-area atomically thin MoS2 film into a hybrid graphene photoconductor, we have developed a prototype tunneling photoconductor, which exhibits a record-fast response (rising time ~17 ns) and a high responsivity (~3 × 104 A/W at 635 nm illumination with 16.8 nW power) across the broad spectral range. We demonstrate that the photo-excited carriers generated in silicon are transferred into graphene through a tunneling process rather than carrier drift. The atomically thin MoS2 film not only serves as tunneling layer but also passivates surface states, which in combination delivers a superior response speed (~3 orders of magnitude improved than a device without MoS2 layer), while the responsivity remains high. This intriguing tunneling photoconductor integrates both fast response and high responsivity and thus has significant potential in practical applications of optoelectronic devices. Graphene-based photodetectors with an embedded MoS2 tunnel layer show remarkable responsivities, whilst still retaining fast response times. A team led by Jian-Bin Xu at the Chinese University of Hong Kong tackled the trade-off between speed and responsivity by intercalating two-dimensional MoS2 into a graphene photodetector. This results in the formation of a hybrid tunneling photoconductor, where silicon plays the role of optically active layer, whereas MoS2 serves as tunneling layer. The insertion of ultra-thin MoS2 enables fast transfer of the photo-excited carriers in silicon towards graphene, whilst also passivating surface states. This approach effectively bypasses the speed limitations caused by the long lifetime of trapped interfacial carriers, resulting in a remarkable 17 ns response time and a high, broadband responsivity up to 3 × 104 A/W.

Hybrid graphene tunneling photoconductor with interface engineering towards fast photoresponse and high responsivity

DOI: 10.1002/adom.201801250 layer-dependent bandgap,[10] valley selective circular dichroism,[11] high on/off ratio,[12] and high thermostability[12] of the 2D MoS2. The performance of these novel devices significantly depends on the intrinsic optical properties (especially the dielectric function) of the 2D MoS2, which exhibit an intriguing layer dependency due to the enhanced quantum confinement effect and the absence of inversion symmetry. Therefore, the effective characterization of the layer-dependent optical properties of MoS2 is critical for the performance improvement and the optimal design of those photoelectric devices based on MoS2. Layer-dependent optical properties of MoS2, including absorbance,[1] photoluminescence spectra,[10] Raman spectra,[13] and second-harmonic generation effect,[14] have been measured and discussed previously. However, these studies can hardly gain the basic optical constants, such as dielectric function, complex refraction index, etc., which play an important role in the quantitative design and optimization of those MoS2-based photoelectric devices.[15] Beyond these researches, there are also some reports on the dielectric function of the 2D MoS2, where the techniques they used can be roughly divided into three types: ellipsometry,[16–26] reflection (or absorption) spectrum method,[27,28] and contrast spectrum or differential reflection (or transmission) spectrum method.[29,30] By using of ellipsometry, Li et al. investigated the optical properties of the monolayer and bulk MoS2, and identified some critical points (CPs) in their dielectric function spectra.[22] The frequency-dependent reflection (transmission) spectra and corresponding differential spectra of the monolayer MoS2 were simultaneously measured by Morozov et al., then the dielectric function was extracted.[27] Li et al. measured the reflection spectra of the mechanical exfoliation monolayer MoS2 flake and the bulk MoS2, then their dielectric functions were calculated combined with a Kramers–Kronig constrained variational analysis.[28] Nevertheless, these published experimental reports on the dielectric function of MoS2 mainly focus on the monolayer and bulk counterpart and the spectral range is relatively narrow. Apart from these experimental studies, some researchers have also devoted to predict the dielectric properties of the 2D MoS2 with the help of theoretical calculations.[31,32] For example, Johari et al. studied the dielectric properties of monolayer, bilayer, and bulk MoS2 by computing the electron Wafer-scale, high-quality, and layer-controlled 2D MoS2 films on c-sapphire are synthesized by an innovative two-step method. The dielectric functions of MoS2 ranging from the monolayer to the bulk are investigated by spectroscopic ellipsometry over an ultra-broadband (0.73–6.42 eV). Up to five critical points (CPs) in the dielectric function spectra are precisely distinguished by CP analysis, and their physical origins are identified in the band structures with the help of first-principles calculations. Results demonstrate that the center energies of these CPs exhibit intriguing layer dependency, which are interpreted by the intrinsic layer-dependent transitions in MoS2. Specially, the change in the imaginary part of the dielectric functions versus the thickness exhibits a “W” like curve, and the two valley bottoms appear at about fourlayer and 10-layer respectively. These complex fluctuations are attributed to the alternating domination of the decreasing excitonic effect, the increasing joint density of states, and the mass density increase in relative thick MoS2 samples.

Layer-Dependent Dielectric Function of Wafer-Scale 2D MoS2

Chiral Coupling of Valley Excitons and Light through Photonic Spin–Orbit Interactions

Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A−1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors.

Photoconversion of carbon dioxide with water into renewable solar fuels using a single integrated system is the primary goal of artificial photosynthesis The ease of applying mechanical strain reversibly in low dimensional materials can be exploited in optimizing their artificial photosynthetic or photocatalytic properties In this work, strain effects on the concerted interplay of all the factors controlling the photocatalytic ability, such as suitable band edges straddling the redox potentials for CO2 reduction with the water and oxygen evolution reactions, optical absorbance and conductivity, carrier mobilities, carrier mobility ratios (ie hole/electron or electron/hole mobility ratio) and exciton binding energies, have been comprehensively studied on monolayer MoS2 nanosheets and calibrated, using density functional theory, to ascertain the type and magnitude of strain under which the optical absorbance and conductivity, carrier mobilities and the carrier mobility ratio are simultaneously and optimally enhanced, while the requisite redox reactivity and small exciton binding energies are retained Additionally, the role of pH has been carefully addressed Optical conductivity in pristine ML-MoS2 calculated in this work agrees with the experimental ones Of all strain types studied, shear strain is found to be most effective in driving the kinetics of the light-driven reduction of CO2 with water, water reduction in the absence of CO2 [ie the H2 evolution reaction (HER)] and oxidation of water [ie, the O2 evolution reaction (OER)] with nearly the same efficiency The optical absorbance and conductivity are found to reach their maximum magnitude at 3% shear strain along the direction of lattice dilation 05–35 and 05–65 ranges in pH are found to be favorable for the photo-catalytic HER and CO2 reduction with water respectively Carrier mobility has been calculated using a more reliable and robust formulation where the carrier mobility depends not only on the carrier effective mass, deformation potential and elastic modulus along the longitudinal charge transport direction but also on these parameters along the transverse direction Shear strain induces the maximum enhancement in carrier mobilities and the carrier mobility ratio, which facilitates the separation and migration of e−–h+ pairs along opposite or transverse directions and thereby in prolonging the carrier lifetimes, ie, lowering their recombination rates

Strain and pH facilitated artificial photosynthesis in monolayer MoS2 nanosheets

Optical conductivity plays an important role in characterizing the optoelectronic properties of two-dimensional materials. Here we derive the complex optical conductivities for monolayer and few-layered MoS2 films from their reflectance and transmittance responses. We show that the excitonic quantum confinement effect significantly modifies both the peak energy and magnitude of their optical conductivity, manifested by a gradual blueshift in energy (consistent with two well-known models for quantum well systems) and exponential attenuation in magnitude with decreasing layer number. More importantly, the C excition induced optical conductivity peak exhibits the strongest dependence on the MoS2 layer number because of its largest Bohr radius among the A, B and C excitons. This unambiguously confirms the strong influence of quantum confinement effect in the optical conductivity of MoS2, shedding important insights into understanding its rich exciton-related optical properties and therefore facilitating potential applications in optoelectronic devices.

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Excitonic quantum confinement modified optical conductivity of monolayer and few-layered MoS2

Atomically thin transition metal dichalcogenides (TMDCs) exhibit fascinating excitonic properties which are of critical importance for potential applications in nanoscale optoelectronics. In particular, stacked TMDC layers without lattice mismatch allow modulation of their band structures and optoelectronic properties through manipulation of the constituent materials along the stacking direction. Current understanding of TMDC layer-by-layer structures is mainly based on optical measurements and DFT calculations. In this work, we use a phenomenological tight-binding model, combined with DFT calculations, to understand the measured layer-dependent excitonic response of WS2. This explicit and effective model can quantitatively predict the layer-dependent excitonic states and can also be used to study interlayer excitons (direct and indirect exciton states) in TMDC multilayer structures. In addition, we find that the temperature dependence of the A exciton emission energy of monolayer WS2 can be well described with the Varshni formula, and that the emission intensity variation with temperature is associated with thermal redistribution of exciton population and increased non-radiative damping due to the enhanced electron–phonon interaction at elevated temperatures.

Guang Yi Jia

Papers

Excitonic quantum confinement modified optical conductivity of monolayer and few-layered MoS2

Tight-binding modeling of excitonic response in van der Waals stacked 2D semiconductors