A new memory structure using threshold shifting from charge stored in nanocrystals of silicon (≊5nm in size) is described. The devices utilize direct tunneling and storage of electrons in the nanocrystals. The limited size and capacitance of the nanocrystals limit the numbers of stored electrons. Coulomb blockade effects may be important in these structures but are not necessary for their operation. The threshold shifts of 0.2–0.4 V with read and write times less than 100’s of a nanosecond at operating voltages below 2.5 V have been obtained experimentally. The retention times are measured in days and weeks, and the structures have been operated in an excess of 109 cycles without degradation in performance. This nanomemory exhibits characteristics necessary for high density and low power.

In this letter, we present non-degenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS 2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼2 ps and a slow time scale that lasts longer than ∼100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two dimensional atomic materials, together with strong electron and hole correlations in two dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two dimensional atomic materials. Electron-Hole Recombination Dynamics in Monolayer MoS 2Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as interesting materials both from the perspective of basic science as well as applications [1][2][3][4][5][6][7] . Applications of these materials in electronics and optoelectronics have been extensively explored in recent years 5,6,[8][9][10][11][12][13][14][15][16][17][18] . arXiv:1409.4518v1 [cond-mat.mes-hall] 16 Sep 20142 The bandgaps of most TMDs are in the visible to near-IR wavelength range, making these materials suitable for light emitters, photodetectors, and solar cells 5,10,13,[16][17][18] . In addition, optical control of valley polarization in TMDs has opened opportunities for devices based on the valley degree of freedom 6 . The lifetimes of electrons and holes are critical to all the proposed and demonstrated TMD optoelectronic devices. Despite the recent progress, carrier lifetimes and nonradiative electron-hole recombination mechanisms in TMDs remain poorly understood. Developing a better understanding of the non-radiative electron-hole recombination mechanisms in TMDs is especially important because the reported quantum efficiencies in both TMD light emitters and detectors are extremely poor; in the .0001-.01 range 10,13,[16][17][18] . Similar quantum efficiencies for TMDs have been observed in photoluminescence experiments 2,3 . In contrast, the best reported internal and external quantum efficiencies observed in photoluminescence in III-V semiconductors exceed 0.9 and 0.7, respectively 19 . Therefore, most of the electrons and holes injected either electrically or optically i...

Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent.Graphene is a 2D lattice of carbon atoms arranged in a honeycomb crystal structure with a zero (or nearzero) bandgap and a linear energy-momentum dispersion relation for both electrons and holes [1,2]. The unique electronic and optical properties of graphene make it a promising material for the development of high-speed electron devices, including field-effect transistors, pn-diodes, Terahertz oscillators, and electronic and optical sensors [2,3,4,5,6,7]. The realization of graphene-based devices requires understanding the nonequilibrium carrier dynamics as well as the rate at which electron-hole recombination occurs.Measurements of the ultrafast intraband relaxation dynamics of photogenerated electrons and holes in epitaxial graphene using both degenerate [8] and non-degenerate [9] optical-pump optical-probe spectroscopy have been previously reported. Similar measurements for exfoliated graphene mono-and multi-layers have also been carried out [10]. These measurements were sensitive to the interband conductivity of graphene and probed the time evolution of the carrier occupation at specific energies in the bands. Consequently, they were not able to directly measure the time scales associated with carrier recombination. At room temperature, the optical response of graphene in the THz frequency range is described by the intraband conductivity -the free carrier responsewhich depends not only on the total carrier concentration but also on the carrier distribution in energy [11]. Therefore, THz radiation can be used to study the carrier relaxation and recombination dynamics in graphene. In this paper, we present results obtained from opticalpump THz-probe spectroscopy of epitaxial graphene in which the time-dependent conductivity of graphene that has been excited with an optical pump pulse is probed with a few-cycle THz pulse. We observe cooling of the photogenerated carrier distribution as well as electronhole recombination in graphene in real time. Our results indicate that the recombination times in graphene depend on the carrier density and material disorder.The epitaxial graphene samples used in this work were grown on the carbon face of semi-insulating 6H-SiC wafers using techniques that have been reported previously [12]. As discussed in [8,11], X-ray photoemission, Raman, and optical/IR/THz transmission spectroscopy ...

Abstract-In this paper we propose and discuss coherent terahertz sources based on charge density wave (plasmon) amplification in two dimensional graphene. The coupling of the plasmons to interband electron-hole transitions in population inverted graphene layers can lead to plasmon amplification through stimulated emission. Plasmon gain values in graphene can be very large due to the small group velocity of the plasmons and the strong confinement of the plasmon field in the vicinity of the graphene layer. We present a transmission line model for plasmon propagation in graphene that includes plasmon dissipation and plasmon interband gain due to stimulated emission. Using this model, we discuss design for terahertz plasmon oscillators and derive the threshold condition for oscillation taking into account internal losses and also losses due to external coupling. The large gain values available at terahertz frequencies in graphene can lead to integrated oscillators that have dimensions in the 1-10 µm range.

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent carrier cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.

We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz (THz) frequency range. In the THz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of e 2 /4h. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements.

We measure the optical absorption spectra and optical conductivities of excitons and trions in monolayers of metal dichalcogenide MoS2 and compare the results with theoretical models. Our results show that the Wannier-Mott model for excitons with modifications to account for small exciton radii and large exciton relative wavefunction spread in momentum space, phase space blocking due to Pauli exclusion in doped materials, and wavevector dependent dielectric constant gives results that agree well with experiments. The measured exciton optical absorption spectra are used to obtain experimental estimates for the exciton radii that fall in the 7 − 10Å range and agree well with theory. The measured trion optical absorption spectra are used to obtain values for the trion radii that also agree well with theory. The measured values of the exciton and trion radii correspond to binding energies that are in good agreement with values obtained from first principles calculations.