There are various mechanisms of light emission in carbon nanotubes (CNTs), which give rise to a wide range of spectral emission characteristics that provide important information regarding the underlying physical processes that lead to photon emission. Here, we report spectra obtained from individual suspended CNT dual-gate field effect transistor (FET) devices under different gate and bias conditions. By applying opposite voltages to the gate electrodes (i.e., V_g1 = -V_g2), we are able to create a pn-junction within the suspended region of the CNT. Under forward bias conditions, the spectra exhibit a peak corresponding to E₁₁ exciton emission via thermal (i.e., blackbody) emission occurring at electrical powers around 8 µW, which corresponds to a power density of approximately 0.5 MW/cm². On the other hand, the spectra observed under reverse bias correspond to impact ionization and avalanche emission, which occurs at electrical powers of ~ 10 nW and exhibits a featureless flat spectrum extending from 1,600 nm to shorter wavelengths up to 600 nm. Here, the hot electrons generated by the high electric fields (~ 0.5 MV/cm) are able to produce high energy photons far above the E₁₁ (ground state) energy. It is somewhat surprising that these devices do not exhibit light emission by the annihilation of electrons and holes under forward bias, as in a light emitting diode (LED). Possible reasons for this are discussed, including Auger recombination.

We report plasmon resonant excitation of hot electrons in a photodetector based on a metal/oxide/metal (Au/Al₂O₃/graphene) heterostructure. In this device, hot electrons, excited optically in the gold layer, jump over the oxide barrier and are injected into the graphene layer, producing a photocurrent. To amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent produced in this device is measured using 633-nm-wavelength light as a function of incident angle. We observe the maximum photocurrent at ±10° from normal incidence under irra-diation with light polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating, and a constant (angle-independent) photocurrent under irradiation with light polarized perpendicular to the incident plane (s-polarization) and parallel to the grating. These data show an amplification factor of 4.6× under resonant conditions. At the same angle (±10°), we also observe sharp dips in the photoreflectance corresponding to waveve-ctor matching between the incident light and the plasmon mode in the grating. In addition, finite-difference time-domain simulations predict sharp dips in the photoreflectance at ±10°, and the electric field intensity profiles show clear excitation of a plasmon resonant mode when illuminated with p-polarized light at this angle.

We report thermoelectric transport measurements across a graphene/hexagonal boron nitride (h-BN)/graphene heterostructure device. Using an AC lock-in technique, we are able to separate the thermoelectric contribution to the I–V characteristics of these important device structures. The temperature gradient is measured optically using Raman spectroscopy, which enables us to explore thermoelectric transport produced at material interfaces, across length scales of just 1–2 nm. Based on the observed thermoelectric voltage (ΔV) and temperature gradient (ΔT), a Seebeck coefficient of -99.3 μV/K is ascertained for the heterostructure device. The obtained Seebeck coefficient can be useful for understanding the thermoelectric component in the cross-plane I–V behaviors of emerging 2D heterostructure devices. These results provide an approach to probing thermoelectric energy conversion in two-dimensional layered heterostructures.

We fabricate arrays of metallic nanoparticle dimers with nanometer separation using electron beam lithography and angle evaporation. These "nanogap" dimers are fabricated on thin silicon nitride membranes to enable high resolution transmission electron microscope imaging of the specific nanoparticle geometries. Plasmonic resonances of the pairs are characterized by dark-field scattering micro-spectroscopy, which enables the optical scattering from individual nanostructures to be measured by using a spatially-filtered light source to illuminate a small area. Scattering spectra from individual dimers are correlated with transmission electron microscope images and finite-difference time-domain simulations of their electromagnetic response, with excellent agreement between simulation and experiment. We observe a strong polarization dependence with two dominant scattering peaks in spectra taken with the polarization aligned along the dimer axis. This response arises from a unique Fano interference, in which the bright hybridized modes of an asymmetric dimer are able to couple to the dark higherorder hybridized modes through substrate-mediated coupling. The presence of this interference is strongly dependent on the nanoparticle geometry that defines the plasmon energy profile but also on the intense localization of charge at the dielectric surface in the nanogap region for separations smaller than 6 nm.

We report substantial improvements in the photoluminescence (PL) efficiency and Fabry-Perot (FP) resonance of individual GaAs nanowires through surface passivation and local field enhancement, enabling FP peaks to be observed even at room temperature. For bare GaAs nanowires, strong FP resonance peaks can be observed at 4 K, but not at room temperature. However, depositing the nanowires on gold substrates leads to substantial enhancement in the PL intensity (5X) and 3.7X to infinite enhancement of FP peaks. Finite-difference time-domain (FDTD) simulations show that the gold substrate enhances the PL spectra predominately through enhanced absorption (11X) rather than enhanced emission (1.3X), predicting a total PL enhancement of 14X in the absence of non-radiative recombination. Despite the increased intensity of the FP peaks, lower Q factors are observed due to losses associated with the underlying gold substrate. As a means of reducing the non-radiative recombination in these nanowires, the surface states in the nanowires can be passivated by either an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI)) or an AlGaAs surface layer to achieve up to 12X enhancement of the photoluminescence intensity and observation of FP peaks at room temperature without a gold substrate.

We report substantial improvements and modulation in the photocurrent (PC) and photoluminescence (PL) spectra of monolayer MoS₂ recorded under electrostatic and ionic liquid gating conditions. The photocurrent and photoluminescence spectra show good agreement with a dominant peak at 1.85 eV. The magnitude of the photoluminescence can be increased 300% by ionic liquid gating due to the passivation of surface states and trapped charges that act as recombination centers. The photocurrent also doubles when passivated by the ionic liquid. Interestingly, a significant shift of the PL peak position is observed under electrostatic (14 meV) and ionic liquid (30 meV) gating, as a result of passivation. The ionic liquid provides significant screening without any externally applied voltage, indicating that these surface recombination centers have net charge. The acute sensitivity of monolayer MoS₂ to ionic liquid gating and passivation arises because of its high surface-to-volume ratio, which makes it especially sensitive to trapped charge and surface states. These results reveal that, in order for efficient optoelectronic devices to be made from monolayer MoS₂, some passivation strategy must be employed to mitigate the issues associated with surface recombination.

We have recently demonstrated that GaAs nanosheets can be grown by metal-organic chemical vapor deposition (MOCVD). Here, we investigate these nanosheets by secondary electron scanning electron microscopy (SE-SEM) and electron beam induced current (EBIC) imaging. An abrupt boundary is observed between an initial growth region and an overgrowth region in the nanosheets. The SE-SEM contrast between these two regions is attributed to the inversion of doping at the boundary. EBIC mapping reveals a p–n junction formed along the boundary between these two regions. Rectifying Ⅰ–Ⅴ behavior is observed across the boundary further indicating the formation of a p–n junction. The electron concentration (ND) of the initial growth region is around 1 × 10¹⁸ cm^–3, as determined by both Hall effect measurements and low temperature photoluminescence (PL) spectroscopy. Based on the EBIC data, the minority carrier diffusion length of the nanosheets is 177 nm, which is substantially longer than the corresponding length in unpassivated GaAs nanowires measured previously.

We report the emergence of the D band Raman mode in single-walled carbon nanotubes under large axial strain. The D to G mode Raman intensity ratio (I_D/I_G) is observed to increase with strain quadratically by more than a factor of 100-fold. Up to 5% strain, all changes in the Raman spectra are reversible. The emergence of the D band, instead, arises from the reversible and elastic symmetry-lowering of the sp² bonds structure. Beyond 5%, we observe irreversible changes in the Raman spectra due to slippage of the nanotube from the underlying substrate, however, the D band intensity resumes its original pre-strain intensity, indicating that no permanent defects are formed.

Electrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or "kink"). The bias voltage at the kink (V_kink) is found to depend strongly on gate voltage, substrate temperature, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2 V, independent of gate voltage and gas environment. This bias voltage of 0.2 V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the G band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system.