We observe bright electroluminescence from suspended carbon nanotube (CNT) field effect transistors (FETs) under extremely low applied electrical powers (∼nW). Here, light emission occurs under positive applied gate voltages, with the FET in its “off” state. This enables us to apply high bias voltages (4 V) without heating the CNT. Under these conditions, we observe light emission at currents as small as 1 nA and corresponding electrical powers of 4nW, which is 3 orders of magnitude lower than previous studies. Thermal emission is ruled out by monitoring the G band Raman frequency, which shows no evidence of heating under these small electrical currents. The mechanism of light emission is understood on the basis of steep band bending that occurs in the conduction and valence band profiles at the contacts, which produces a peak electric field of 500 kV/cm, enabling the acceleration of carriers beyond the threshold of exciton emission. The exciton-generated electrons and holes are then accelerated in this field and emit excitons in an avalanche process. This is evidenced by an extremely sharp increase in the current with bias voltage (45 mV/dec). We also observe light emission at negative applied gate voltages when the FET is in its “on” state at comparable electrical powers to those reported previously (∼5 μW). However, substantial Joule heating (T > 1000 K) is also observed under these conditions, and it is difficult to separate the mechanisms of thermal emission from hot carrier photoemission in this regime.
Using hot electrons to drive electrochemical reactions has drawn considerable interest in driving high-barrier reactions and enabling efficient solar to fuel conversion. However, the conversion efficiency from hot electrons to electrochemical products is typically low due to high hot electron scattering rates. Here, it is shown that the hydrogen evolution reaction (HER) in an acidic solution can be efficiently modulated by hot electrons injected into a thin gold film by an Au–Al2O3–Si metal–insulator–semiconductor (MIS) junction. Despite the large scattering rates in gold, it is shown that the hot electron driven HER can reach quantum efficiencies as high as ∼85% with a shift in the onset of hydrogen evolution by ∼0.6 V. By simultaneously measuring the currents from the solution, gold, and silicon terminals during the experiments, we find that the HER rate can be decomposed into three components: (i) thermal electron, corresponding to the thermal electron distribution in gold; (ii) hot electron, corresponding to electrons injected from silicon into gold which drive the HER before fully thermalizing; and (iii) silicon direct injection, corresponding to electrons injected from Si into gold that drive the HER before electron–electron scattering occurs. Through a series of control experiments, we eliminate the possibility of the observed HER rate modulation coming from lateral resistivity of the thin gold film, pinholes in the gold, oxidation of the MIS device, and measurement circuit artifacts. Next, we theoretically evaluate the feasibility of hot electron injection modifying the available supply of electrons. Considering electron–electron and electron–phonon scattering, we track how hot electrons injected at different energies interact with the gold–solution interface as they scatter and thermalize. The simulator is first used to reproduce other published experimental pump–probe hot electron measurements, and then simulate the experimental conditions used here. These simulations predict that hot electron injection first increases the supply of electrons to the gold–solution interface at higher energies by several orders of magnitude and causes a peaked electron interaction with the gold–solution interface at the electron injection energy. The first prediction corresponds to the observed hot electron electrochemical current, while the second prediction corresponds to the observed silicon direct injection current. These results indicate that MIS devices offer a versatile platform for hot electron sources that can efficiently drive electrochemical reactions.
Photocathodes exhibiting simultaneous high quantum efficiency, low mean transverse energy (MTE), and fast temporal response are critical for next generation electron sources. Currently, caesiated negative electron affinity GaAs photocathodes have demonstrated good overall results [Bell and Spicer, Proc. IEEE 58, 1788 (1970); Pierce et al., Appl. Phys. Lett. 26, 670 (1975)]. However, due to the nature of the photoemission process and the details of the Cs surface structure, a tradeoff exists. A low mean transverse energy of ∼25 meV can be obtained by using photons with near bandgap energy, at the cost of an unacceptably high response time, or higher energy photons can be used with a mean transverse energy of ∼60 meV with acceptable response times of 2–5 ps [Karkare et al., J. Appl. Phys. 113, 104904 (2013); Honda et al., Jpn. J. Appl. Phys. 52, 086401 (2013); Pastuszka et al. Appl. Phys. Lett. 71, 2967 (1997)]. Here, it is shown through a calibrated simulation that a thin layer of caesiated GaAs on a waveguide can potentially exhibit photoemission with MTEs ∼30 meV, ultrafast response times of ∼0.2–1 ps, and quantum efficiency of 1%–10%, breaking the traditional tradeoffs associated with bulk negative electron affinity photoemitters.
We present a systematic study of photoluminescence (PL) spectroscopy of TiO2-passivated GaAs as a function of electrochemical potential in an ionic liquid solution. We observe a 7X increase in the PL intensity as the GaAs transitions from accumulation to depletion due to the applied potential. We attribute this to the excellent control over the surface Fermi level enabled by the high capacitance of the electrochemical double layer and TiO2. This allows us to control the surface carrier concentration and corresponding non-radiative recombination rate. In addition to photoluminescence (PL) spectroscopy, we also measured the capacitance-potential (i.e., C-V) characteristics of these samples, which indicate flat band potentials that are consistent with these regimes of ion accumulation observed in the photoluminescence measurements. We have also performed electrostatic simulations of these C-V characteristics, which provide a detailed and quantitative picture of the conduction and valence band profiles and charge distribution at the surface of the semiconductor. These simulations also enable us to determine the range of potentials over which the semiconductor surface experiences depletion, inversion, and accumulation of free carriers. Based on these simulations, we can calculate the Shockley-Read-Hall recombination rate and model the PL intensity as a function of voltage. We show that this approach allows us to explain our experimental data well.
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