Nathaniel M Gabor

Massachusetts Institute of Technology | MIT · Department of Physics

Photocurrent in quantum materials is often collected at global contacts far away from the initial photoexcitation. This collection process is highly nonlocal. It involves an intricate spatial pattern of photocurrent flow (streamlines) away from its primary photoexcitation that depends sensitively on the configuration of current collecting contacts...

Stack engineering, an atomic-scale metamaterial strategy, enables the design of optical and electronic properties in van der Waals heterostructure devices. Here we reveal the optoelectronic effects of stacking-induced strong coupling between atomic motion and interlayer excitons in WSe2/MoSe2 heterojunction photodiodes. To do so, we introduce the p...

We have measured the reflectance contrast, photoluminescence, and valley polarization of a WSe_{2}/WS_{2} heterobilayer moiré superlattice at gate-tunable charge density. We observe absorption modulation of three intralayer moiré excitons at filling factors ν=1/3 and 2/3. We also observe luminescence modulation of interlayer trions at around a doze...

Moiré superlattices formed by van der Waals materials can support a wide range of electronic phases, including Mott insulators1–4, superconductors5–10 and generalized Wigner crystals2. When excitons are confined by a moiré superlattice, a new class of exciton emerges, which holds promise for realizing artificial excitonic crystals and quantum optic...

We report the direct observation of intervalley exciton between the Q conduction valley and $\Gamma$ valence valley in bilayer WSe$_2$ by photoluminescence. The Q$\Gamma$ exciton lies at ~18 meV below the QK exciton and dominates the luminescence of bilayer WSe$_2$. By measuring the exciton spectra at gate-tunable electric field, we reveal differen...

We reveal stacking-induced strong coupling between atomic motion and interlayer excitons through photocurrent measurements of WSe$_2$/MoSe$_2$ heterojunction photodiodes. Strong coupling manifests as pronounced periodic sidebands in the photocurrent spectrum in frequency windows close to the interlayer exciton resonances. The sidebands, which repea...

Moir\'e superlattices are excellent platforms to realize strongly correlated quantum phenomena, such as Mott insulation and superconductivity. In particular, recent research has revealed stripe phases and generalized Wigner crystals at fractional fillings of moir\'e superlattices. But these experiments have not focused on the influence of electroni...

Capturing and converting solar energy into fuels and feedstocks is a global challenge that spans numerous disciplines and fields of research. Billions of years of evolution have allowed natural organisms to hone strategies for harvesting light from the sun and storing energy in the form of carbon–carbon and carbon–hydrogen bonds. Photosynthetic ant...

Pairs of peaks stabilize output power A counterintuitive feature of photosynthesis is that the primary pigments involved in absorbing light—for example, chlorophyll a and b in plants—do not all absorb right at the peak of the spectrum but instead are offset from the peak and each other. Arp et al. formulated a network model that explains how using...

Excitons and trions (or exciton polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violat...

Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violat...

Like air flowing over a wing, optimizing the flow of electronic charge is essential to the operation of nanoscale devices. Unfortunately, the delicate interplay of charge, spin, and heat in complex devices has precluded detailed imaging of charge flow. Here, we report on the visualization of intrinsic charge current streamlines through yttrium iron...

Photosynthesis is remarkable, achieving near unity light harvesting quantum efficiency in spite of dynamic light conditions and noisy physiological environment. Under these adverse conditions, it remains unknown whether there exists a fundamental organizing principle that gives rise to robust photosynthetic light harvesting. Here, we present a nois...

In semiconductors, photo-excited charge carriers exist as a gas of electrons and holes, bound electron–hole pairs (excitons), biexcitons and trions1–4. At sufficiently high densities, the non-equilibrium system of electrons (e⁻) and holes (h⁺) may merge into an electronic liquid droplet5–10. Here, we report on the electron–hole liquid in ultrathin...

When the Fermi level matches the Dirac point in graphene, the reduced charge screening can dramatically enhance electron-electron (e-e) scattering to produce a strongly interacting Dirac liquid. While the dominance of e-e scattering already leads to novel behaviors, such as electron hydrodynamic flow, further exotic phenomena have been predicted to...

Quantum devices made from van der Waals (vdW) heterostructures of two dimensional (2D) materials may herald a new frontier in designer materials that exhibit novel electronic properties and unusual electronic phases. However, due to the complexity of layered atomic structures and the physics that emerges, experimental realization of devices with ta...

Quantum devices made from van der Waals (vdW) heterostructures of two dimensional (2D) materials may herald a new frontier in designer materials that exhibit novel electronic properties and unusual electronic phases. However, due to the complexity of layered atomic structures and the physics that emerges, experimental realization of devices with ta...

In recent decades, scientists have developed the means to engineer synthetic periodic arrays with feature sizes below the wavelength of light. When such features are appropriately structured, electromagnetic radiation can be manipulated in unusual ways, resulting in optical metamaterials whose function is directly controlled through nanoscale struc...

Ultrafast electron thermalization - the process leading to Auger recombination, carrier multiplication via impact ionization and hot carrier luminescence - occurs when optically excited electrons in a material undergo rapid electron-electron scattering to redistribute excess energy and reach electronic thermal equilibrium. Due to extremely short ti...

In this work, we leverage graphene's unique tunable Seebeck coefficient for the demonstration of a graphene-based thermal imaging system. By integrating graphene based photo-thermo-electric detectors with micro-machined 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...

We report on temperature dependent photocurrent measurements of high-quality dual-gated monolayer graphene (MLG) p-n junction devices. A photothermoelectric (PTE) 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 K to 30...

We explore the photoresponse of an ambipolar graphene infrared thermocouple at photon energies close to or below monolayer graphene's 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 i...

Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since its discovery in 2004. Due to its zero band gap band structure, it has a broad-band optical absorption ranging from the far-infrared all the way to the visible making it potentially useful for infrared photodetectors. Electrostatically gated p...

Monolayer graphene, an atomically thin sheet of hexagonally oriented carbon, is a zero band gap conductor that exhibits strong electron-electron interactions and broadband optical absorption. By combining MLG and hexagonal boron nitride into ultrathin vertical stacks, experiments have demonstrated improved mobility, Coulomb drag, and field-effect t...

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 e...

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...

Optoelectronic devices composed of atomically thin graphene and boron nitride membranes yield great promise for next-generation photonics and optoelectronic research, yet numerous fabrication challenges remain. We use chemical vapor deposited (CVD) graphene to produce atomically thin, local bottom-gates for high-quality exfoliated graphene optoelec...

We report on temperature dependent photocurrent measurements of high-quality dual-gated monolayer graphene p-n junction devices. Over temperatures ranging from 5 K to 300 K, we find that the photocurrent at the p-n interface peaks at an intermediate temperature, and decreases at higher and lower temperatures. Spatial photocurrent microscopy (at wav...

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 spat...

Ultrafast photocurrent measurements are performed on individual carbon nanotube PN junction photodiodes. The photocurrent response to sub-picosecond pulses separated by a variable time delay {\Delta}t shows strong photocurrent suppression when two pulses overlap ({\Delta}t = 0). The picosecond-scale decay time of photocurrent suppression scales inv...

The appearance of mathematical regularities in the disposition of leaves on a stem, scales on a pine-cone, and spines on a cactus has puzzled scholars for millennia; similar so-called phyllotactic patterns are seen in self-organized growth, polypeptides, convection, magnetic flux lattices and ion beams. Levitov showed that a cylindrical lattice of...

We have built crossed carbon nanotube/graphene junctions from CVD graphene and aligned arrays of carbon nanotubes. Large-area single-layer graphene was grown on a copper film and transferred to silicon oxide, then lithographically patterned and electrically contacted. Highly aligned arrays of single-walled carbon nanotubes were CVD-grown on quartz...

We investigate the optoelectronic response of a graphene interface junction, formed with bilayer and single-layer graphene, by 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 photo-thermoelectric effect. Th...

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. Intere...

While the statics of many simple physical systems reproduce the striking number-theoretical patterns found in the phyllotaxis of living beings, their dynamics reveal unusual excitations: multiple classical rotons and a large family of interconverting topological solitons. As we introduce those, we also demonstrate experimentally for the first time...

We describe the first terahertz electrical measurements of single-walled carbon nanotube transistors. A picosecond-ballistic-electron resonance is directly observed in the time-domain. These results demonstrate a powerful new tool for directly probing picosecond-electron motion in nanostructures.

Phyllotaxis, the study of mathematical patterns in the arrangement of leaves on stems, spines on cacti, petals on flowers, et cetera, fascinated mankind since the dawn of times. Similar patterns emerge in the the statics of simple physical systems. Here we reproduce experimentally the striking number-theoretical patterns found in the phyllotaxis of...

We describe the first terahertz electrical measurements of single-walled carbon nanotube transistors. A picosecond ballistic electron resonance is directly observed in the time-domain. These results demonstrate a powerful new tool for directly probing picosecond electron motion in nanostructures.

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...

We investigate the photocurrent response at a nanotube gated p-n junction using a focused laser illumination source. Scanned photocurrent imaging demonstrates that photocurrent response occurs primarily in the p-n junction. Measurements in an optical cryostat down to 4K reveal large photoresponse and unusual step-like structure in the reverse bias...

Gated p-n junctions in semiconducting nanotubes have recently drawn much attention for their electronic and optoelectronic characteristics [1,2,3]. We investigate the photocurrent response at a nanotube gated p-n junction using a focused laser illumination source. We find that the photocurrent at zero source-drain bias increases linearly with optic...

Single-walled carbon nanotube diodes are fabricated in a split-gate geometry with electron (n) and hole (p) regions separated by a central region. With the central region gated p or n type the diodes “leak” at low voltages, likely due to tunneling across the smaller depletion region. With the central region intrinsic, nearly ideal diode behavior is...

We have investigated electronic transport in single-walled carbon nanotube p-n junction diodes formed using gates to electrostatically dope the tube. Previous measurements [1] have shown that such diodes demonstrate nearly ideal turn-on behavior at room temperature and low biases, consistent with thermal activation over the junction barrier. We hav...

The disposition of the leaves on a stem, spines on a cactus, seeds in a sunflower and other self-organized arrangements of repeated units in plant morphology, are denoted as phyllotaxis. Phyllotactic patterns are also found in polypeptide chains, cells of Bernard convection, and flux lattices in layered superconductors. Here we describe a "magnetic...