The water-soluble fluorescent carbon quantum dots (CQDs) are synthesized by utilizing lemon juice as carbon resource via a simple hydrothermal reaction. The obtained CQDs are with an average size of 3.1 nm. They reveal uniform morphology and well-crystalline and can generate bright blue-green light emission under UV or blue light irradiation. We find that the fluorescence from these CQDs is mainly induced by the presence of oxygen-containing groups on the surface and edge of the CQDs. Moreover, we demonstrate that the as-prepared CQDs can be applied to imaging plant cells. This study is related to the fabrication, investigation, and application of newly developed carbon nanostructures.

A theoretical study of the collective excitation associated with plasmon modes is presented for a two-dimensional electron gas in the presence of spin orbit (SO) interaction induced by the Rashba effect. In such a case, the plasmon excitation can be achieved via intra- and inter-SO electronic transitions. As a result, three branches of the plasmon oscillations can be observed. It is found that inter-SO plasmons depend strongly on sample parameters and, at a long-wavelength limit, are optic-like, in contrast to intra-SO ones. The interesting features of these plasmon modes are examined.

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Recently, black phosphorus quantum dots were fabricated experimentally. Motivated by these experiments, we theoretically investigate the electronic and optical properties of rectangular phosphorene quantum dots (RPQDs) in the presence of an in-plane electric field and a perpendicular magnetic field. The energy spectra and wave functions of RPQDs are obtained numerically using the tight-binding approach. We find edge states within the band gap of the RPQD which are well separated from the bulk states. In an undoped RPQD and for in-plane polarized light, due to the presence of well-defined edge states, we find three types of optical transitions which are between the bulk states, between the edge and bulk states, and between the edge states. The electric and magnetic fields influence the bulk-to-bulk, edge-to-bulk, and edge-to-edge transitions differently due to the different responses of bulk and edge states to these fields.

We study the mobility of Dirac fermions in monolayer graphene on a GaAs substrate, restricted by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (PA) and of the intrinsic deformation potential of acoustical eigen-phonons in graphene (DA). In the high temperature (T ) regime the momentum relaxation rate exhibits the same linear dependence on T but different dependences on the carrier density n, corresponding to the mobility µ ∝ 1/ √ n and 1/n, respectively for the PA and DA scattering mechanisms. In the low T Bloch-Grüneisen regime, the mobility shows the same square-root density dependence, µ ∝ √ n, but different temperature dependences, µ ∝ T −3 and T −4 , respectively for PA and DA phonon scattering.Graphene [1] due to its unique linear chiral electronic dispersion [2] exhibits novel transport properties [3,4] and has great potential as a desirable material for future electronic and optical technologies [2,5]. Momentum relaxation is a key phenomenon that governs transport of Dirac fermions in graphene [6]. It is of practical interest for developing high-speed electronics and in recent years has been extensively studied both theoretically [7][8][9] and experimentally [10][11][12]. The scattering by defects [7,[13][14][15][16], impurities [8,12,[17][18][19][20][21], and phonons [9,11,16,[22][23][24][25][26] have been investigated to determine and control the dominant mechanism that limits the carrier mobility in graphene.In device structures used so far, graphene is often deposited on an oxidized silicon wafer (SiO 2 /Si), which due to various scattering mechanisms imposes constraints on its excellent transport properties observed in suspended graphene devices [10,27]. Recently, structures on other promising substrate materials such as h-BN [28,29] and GaAs [30,31] have been fabricated and studied with the intention for high-quality graphene electronics. Along with its superior surface quality and strong hydrophilicity preventing folding of large-scale graphene flakes, GaAs has a substantially larger dielectric constant in comparison with SiO 2 and h-BN and hence improved electrical screening. In such high purity GaAs structures, electronphonon scattering can be a decisive factor in limiting the mobility of Dirac fermions and the piezoelectric GaAs substrate can serve as a powerful tool for studying electronic properties of graphene by means of remote piezoelectric surface acoustical phonons.In the present work we study the temperature and density dependence of the carrier mobility in monolayer graphene at finite doping on a GaAs substrate. We calculate the mobility limited by scattering from the piezoelectric potential of remote surface acoustical phonons of the substrate (PA phonons) versus the deformation potential of acoustical eigen-phonons of the graphene lattice (DA phonons). In experiment the typical wavelength of phonons taking part in scattering events is much larger than the distance, d, of several angströms between the graphene sheet and the GaAs sub...

We present a detailed theoretical study on nonlinear transport and optical properties in two-dimensional semiconductor systems ͑2DSS's͒ subjected to intense terahertz electromagnetic fields. By solving the momentum-and energy-balance equations using a steady-state Boltzmann equation approach, where electron interaction with LO phonons is taken into account under the lowest-order approximation for GaAs-based 2DSS's, we have investigated the dependence of electron temperature, momentum relaxation time, and conductivity on the intensity and frequency of THz radiation fields. The results obtained from this study are in line with those obtained from recent experimental measurements ͓N. G. Asmar et al., Appl. Phys. Lett. 68, 829 ͑1996͔͒. ͓S0163-1829͑97͒04308-7͔

Low-temperature magnetotransport measurements on GaSb∕InAs∕AlSb coupled quantum well structures with a GaSb cap layer and self-consistent calculations of their electronic structure have led to the determination of the Fermi level at the surface, EFS, of undoped molecular-beam-epitaxy-grown GaSb. EFS is pinned around 0.2eV above the top of the GaSb valence band when the GaSb cap layer width is greater than around 900Å. For smaller GaSb cap widths, EFS decreases with the GaSb width. The undoped GaSb∕InAs∕AlSb heterostructure’s Fermi level is determined by bulk donor defects in the AlSb layer adjacent to the InAs quantum well.

We present a detailed investigation on the effect of functional group modulation at the edges of carbon quantum dots (CQDs) on the fluorescence from the CQDs. The CQDs attached by N, S, and P elements are synthesized via pyrolysis of a mixture of citric acid and NH 3 H 2 O, H 2 SO 4 , and H 3 PO 4 , respectively. Thus, part of –COOH at the edges of CQDs can be converted into –C=O and functional groups such as –NH 2 , –SO 2 , –HSO 3 , and –H 2 PO 4 can connect to the carbon bonds. We find that the formation of the N/S/P-CQDs can reduce the amount of –COOH that attaches to the edges of sp 2 -conjugated π -domains located at centers of these CQDs. This effect can result in the reduction of the non-radiative recombination for electronic transition in these CQDs. As a result, the quantum yield (QY) for fluorescence from the CQDs can be efficiently enhanced. We demonstrate experimentally that the QYs for N/S/P-CQDs can reach up to 18.7%, 29.7%, and 10.3%, respectively, in comparison to 9% for these without functional group modulation. This work can provide a practical experimental approach in improving the optical properties of fluorescent CQDs.