Zehua Jin

Excitation and Active Control of Propagating Surface Plasmon Polaritons in Graphene

We demonstrate the excitation and gate control of highly confined surface plasmon polaritons propagating through monolayer graphene using a silicon diffractive grating. The normal-incidence infrared transmission spectra exhibit pronounced dips due to guided-wave resonances, whose frequencies can be tuned over a range of ~80 cm(-1) by applying a gate voltage. This novel structure provides a way to excite and actively control plasmonic waves in graphene and is thus an important building block of graphene plasmonic systems.

An Atomically Layered InSe Avalanche Photodetector

Atomically thin photodetectors based on 2D materials have attracted great interest due to their potential as highly energy-efficient integrated devices. However, photoinduced carrier generation in these media is relatively poor due to low optical absorption, limiting device performance. Current methods for overcoming this problem, such as reducing contact resistances or back gating, tend to increase dark current and suffer slow response times. Here, we realize the avalanche effect in a 2D material-based photodetector and show that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector. This is achieved by exploiting the large Schottky barrier formed between InSe and Al electrodes, enabling the application of a large bias voltage. Plasmonic enhancement of the photosensitivity, achieved by patterning arrays of Al nanodisks onto the InSe layer, further improves device efficiency. With an external quantum efficiency approaching 866%, a dark current in the picoamp range, and a fast response time of 87 μs, this atomic layer device exhibits multiple significant advances in overall performance for this class of devices.

Tailoring the Physical Properties of Molybdenum Disulfide Monolayers by Control of Interfacial Chemistry

We demonstrate how substrate interfacial chemistry can be utilized to tailor the physical properties of single-crystalline molybdenum disulfide (MoS2) atomic-layers. Semiconducting, two-dimensional MoS2 possesses unique properties that are promising for future optical and electrical applications for which the ability to tune its physical properties is essential. We use self-assembled monolayers with a variety of end termination chemistries to functionalize substrates and systematically study their influence on the physical properties of MoS2. Using electrical transport measurements, temperature-dependent photoluminescence spectroscopy, and empirical and first-principles calculations, we explore the possible mechanisms involved. Our data shows that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS2. These findings can be used to effectively enhance or modulate the conductivity, field-effect mobility, and photoluminescence in MoS2 monolayers, illustrating an approach for local and universal property modulations in two-dimensional atomic-layers.

Plasmonic Nature of the Terahertz Conductivity Peak in Single-Wall Carbon Nanotubes

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and direct current (DC) transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

Surface functionalization of two-dimensional metal chalcogenides by Lewis acid–base chemistry

Precise control of the electronic surface states of two-dimensional (2D) materials could improve their versatility and widen their applicability in electronics and sensing. To this end, chemical surface functionalization has been used to adjust the electronic properties of 2D materials. So far, however, chemical functionalization has relied on lattice defects and physisorption methods that inevitably modify the topological characteristics of the atomic layers. Here we make use of the lone pair electrons found in most of 2D metal chalcogenides and report a functionalization method via a Lewis acid-base reaction that does not alter the host structure. Atomic layers of n-type InSe react with Ti(4+) to form planar p-type [Ti(4+)n(InSe)] coordination complexes. Using this strategy, we fabricate planar p-n junctions on 2D InSe with improved rectification and photovoltaic properties, without requiring heterostructure growth procedures or device fabrication processes. We also show that this functionalization approach works with other Lewis acids (such as B(3+), Al(3+) and Sn(4+)) and can be applied to other 2D materials (for example MoS2, MoSe2). Finally, we show that it is possible to use Lewis acid-base chemistry as a bridge to connect molecules to 2D atomic layers and fabricate a proof-of-principle dye-sensitized photosensing device.

In situ synthesis of polymer-modified mesoporous carbon CMK-3 composites for CO2 sequestration.

Here we report carbon-based composites polyethylenimine-mesocarbon (PEI-CMK-3) and polyvinylamine-mesocarbon (PVA-CMK-3) that can be used to capture and rapidly release CO(2). CO(2) uptake by the synthesized composites was determined using a gravimetric method at 30 °C and 1 atm; the 39% PEI-CMK-3 composite had ~12 wt % CO(2) uptake capacity and the 37% PVA-CMK-3 composite had ~13 wt % CO(2) uptake capacity. A desorption temperature of 75 °C was sufficient for regeneration. The CO(2) uptake was the same when using 10% CO(2) in a 90% CH(4), C(2)H(6), and C(3)H(8) mixture, underscoring this composites efficacy for CO(2) sequestration from natural gas.

Janus Monolayer Transition-Metal Dichalcogenides

The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Circular polarization dependent cyclotron resonance in large-area graphene in ultrahigh magnetic fields

Using ultrahigh magnetic fields up to 170 T and polarized midinfrared radiation with tunable wavelengths from 9.22 to 10.67

Strain-Induced Electronic Structure Changes in Stacked van der Waals Heterostructures.

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Synthesis of large-scale atomic-layer SnS2 through chemical vapor deposition

m, we studied cyclotron resonance in large-area graphene grown by chemical vapor deposition. Circular polarization dependent studies reveal strong