Nathaniel Gabor

Hot carrier-assisted intrinsic photoresponse in graphene.

Photoexcited electrons in graphene remain thermally excited because they cannot transfer this energy to lattice vibrations. We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier–assisted thermoelectric technologies for efficient solar energy harvesting.

Extremely Efficient Multiple Electron-Hole Pair Generation in Carbon Nanotube Photodiodes

Efficient Carbon Nanotube Photodiodes A single photon absorbed in a single-walled carbon nanotube device can generate multiple unbound particles carrying an electric charge. Gabor et al. (p. 1367) report that in such a device at low temperatures, excitation with light of increasing energy leads to well-defined stepwise increases in current. Interestingly, because of the unique band structure of carbon nanotubes, this behavior is analogous to particle-antiparticle creation commonly observed in high-energy particle physics. These observations point to the promise of investigations in other nanoscale carbon systems, such as graphene, and could lead to numerous applications, including highly sensitive photon detection and ultra-efficient photovoltaics. The decay of photoexcited electrons in a carbon nanotube device creates multiple pairs of charge carriers. We observed highly efficient generation of electron-hole pairs due to impact excitation in single-walled carbon nanotube p-n junction photodiodes. Optical excitation into the second electronic subband E22 leads to striking photocurrent steps in the device I-VSD characteristics that occur at voltage intervals of the band-gap energy EGAP/e. Spatially and spectrally resolved photocurrent combined with temperature-dependent studies suggest that these steps result from efficient generation of multiple electron-hole pairs from a single hot E22 carrier. This process is both of fundamental interest and relevant for applications in future ultra-efficient photovoltaic devices.

Photo-Thermoelectric Effect at a Graphene Interface Junction

We investigate the optoelectronic response of a graphene single-bilayer interface junction using photocurrent (PC) microscopy. We measure the polarity and amplitude of the PC while varying the Fermi level by tuning a gate voltage. These measurements show that the generation of PC is by a photothermoelectric effect. The PC displays a factor of approximately 10 increase at the cryogenic temperature as compared to room temperature. Assuming the thermoelectric power has a linear dependence on the temperature, the inferred graphene thermal conductivity from temperature dependent measurements has a T(1.5) dependence below approximately 100 K, which agrees with recent theoretical predictions.

Terahertz time-domain measurement of ballistic electron resonance in a single-walled carbon nanotube

Understanding the physics of low-dimensional systems and the operation of next-generation electronics will depend on our ability to measure the electrical properties of nanomaterials at terahertz frequencies ( approximately 100 GHz to 10 THz). Single-walled carbon nanotubes are prototypical one-dimensional nanomaterials because of their unique band structure and long carrier mean free path. Although nanotube transistors have been studied at microwave frequencies (100 MHz to 50 GHz), no techniques currently exist to probe their terahertz response. Here, we describe the first terahertz electrical measurements of single-walled carbon nanotube transistors performed in the time domain. We observe a ballistic electron resonance that corresponds to the round-trip transit of an electron along the nanotube with a picosecond-scale period. The electron velocity is found to be constant and equal to the Fermi velocity, showing that the high-frequency electron response is dominated by single-particle excitations rather than collective plasmon modes. These results demonstrate a powerful new tool for directly probing picosecond electron motion in nanostructures.

Trion formation dynamics in monolayer transition metal dichalcogenides

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ∼50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.

Photoresponse of an Electrically Tunable Ambipolar Graphene Infrared Thermocouple

We explore the photoresponse of an ambipolar graphene infrared thermocouple at photon energies close to or below monolayer graphenes optical phonon energy and electrostatically accessible Fermi energy levels. The ambipolar graphene infrared thermocouple consists of monolayer graphene supported by an infrared absorbing material, controlled by two independent electrostatic gates embedded below the absorber. Using a scanning infrared laser microscope, we characterize these devices as a function of carrier type and carrier density difference controlled at the junction between the two electrostatic gates. On the basis of these measurements, conducted at both mid- and near-infrared wavelengths, the primary detection mechanism can be modeled as a thermoelectric response. By studying the effect of different infrared absorbers, we determine that the optical absorption and thermal conduction of the substrate play the dominant role in the measured photoresponse of our devices. These experiments indicate a path toward hybrid graphene thermal detectors for sensing applications such as thermography and chemical spectroscopy.

Competing channels for hot-electron cooling in graphene.

We report on temperature-dependent photocurrent measurements of high-quality dual-gated monolayer graphene p-n junction devices. A photothermoelectric effect governs the photocurrent response in our devices, allowing us to track the hot-electron temperature and probe hot-electron cooling channels over a wide temperature range (4 to 300 K). At high temperatures (T > T(*)), we found that both the peak photocurrent and the hot spot size decreased with temperature, while at low temperatures (T < T(*)), we found the opposite, namely that the peak photocurrent and the hot spot size increased with temperature. This nonmonotonic temperature dependence can be understood as resulting from the competition between two hot-electron cooling pathways: (a) (intrinsic) momentum-conserving normal collisions that dominates at low temperatures and (b) (extrinsic) disorder-assisted supercollisions that dominates at high temperatures. Gate control in our high-quality samples allows us to resolve the two processes in the same device for the first time. The peak temperature T(*) depends on carrier density and disorder concentration, thus allowing for an unprecedented way of controlling graphenes photoresponse.

Impact Excitation and Electron–Hole Multiplication in Graphene and Carbon Nanotubes

In semiconductor photovoltaics, photoconversion efficiency is governed by a simple competition: the incident photon energy is either transferred to the crystal lattice (heat) or transferred to electrons. In conventional materials, energy loss to the lattice is more efficient than energy transferred to electrons, thus limiting the power conversion efficiency. Quantum electronic systems, such as quantum dots, nanowires, and two-dimensional electronic membranes, promise to tip the balance in this competition by simultaneously limiting energy transfer to the lattice and enhancing energy transfer to electrons. By exploring the optical, thermal, and electronic properties of quantum materials, we may perhaps find an ideal optoelectronic material that provides low cost fabrication, facile systems integration, and a means to surpass the standard limit for photoconversion efficiency. Nanoscale carbon materials, such as graphene and carbon nanotubes, provide ideal experimental quantum systems in which to explore optoelectronic behavior for applications in solar energy harvesting. Within essentially the same material, researchers can achieve a broad spectrum of energetic configurations, from a gapless semimetal to a large band-gap semiconducting nanowire. Owing to their nanoscale dimensions, graphene and carbon nanotubes exhibit electronic and optical properties that reflect strong electron-electron interactions. Such strong interactions may lead to exotic low-energy electron transport behavior and high-energy electron scattering processes such as impact excitation and the inverse process of Auger recombination. High-energy processes, which become very important under photoexcitation, may be particularly efficient in nanoscale carbon materials due to the relativistic-like, charged particle band structure and sensitivity to the dielectric environment. In addition, due to the covalently bonded carbon framework that makes up these materials, electron-phonon coupling is very weak. In carbon nanomaterials, strong electron-electron interactions combined with weak electron-phonon interactions results in excellent optical, thermal and electronic properties, the exploration of which promises to reveal fundamentally new physical processes and deliver advanced nanotechnologies. In this Account, we review the results of novel optoelectronic experiments that explore the intrinsic photoresponse of carbon nanomaterials integrated into nanoscale devices. By fabricating gate voltage-controlled photodetectors composed of atomically thin sheets of graphene and individual carbon nanotubes, we are able to fully explore electron transport in these systems under optical illumination. We find that strong electron-electron interactions play a key role in the intrinsic photoresponse of both materials, as evidenced by hot carrier transport in graphene and highly efficient multiple electron-hole pair generation in nanotubes. In both of these quantum systems, photoexcitation leads to high-energy electron-hole pairs that relax energy predominantly into the electronic system, rather than heating the lattice. Due to highly efficient energy transfer from photons into electrons, graphene and carbon nanotubes may be ideal materials for solar energy harvesting devices with efficiencies that could exceed the Shockley-Queisser limit.

Graphene-Based Thermopile for Thermal Imaging Applications

In this work, we leverage graphenes unique tunable Seebeck coefficient for the demonstration of a graphene-based thermal imaging system. By integrating graphene based photothermo-electric detectors with micromachined silicon nitride membranes, we are able to achieve room temperature responsivities on the order of ~7-9 V/W (at λ = 10.6 μm), with a time constant of ~23 ms. The large responsivities, due to the combination of thermal isolation and broadband infrared absorption from the underlying SiN membrane, have enabled detection as well as stand-off imaging of an incoherent blackbody target (300-500 K). By comparing the fundamental achievable performance of these graphene-based thermopiles with standard thermocouple materials, we extrapolate that graphenes high carrier mobility can enable improved performances with respect to two main figures of merit for infrared detectors: detectivity (>8 × 10(8) cm Hz(1/2) W(-1)) and noise equivalent temperature difference (<100 mK). Furthermore, even average graphene carrier mobility (<1000 cm(2) V(-1) s(-1)) is still sufficient to detect the emitted thermal radiation from a human target.

Ultrafast Photocurrent Measurement of the Escape Time of Electrons and Holes from Carbon Nanotube p-i-n Photodiodes

Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delay Δt shows strong photocurrent suppression when two pulses overlap (Δt=0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias V(SD), and is twice as long for photon energy above the second subband E22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass.