At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Graphene is a one-atom-thick two-dimensional ͑2D͒ electron system composed of carbon atoms on a honeycomb lattice.1 The lattice has two inequivalent sites in the unit cell that are analogous to the two spin orientations of a spin-1/2 particle. This observation opens the way to an elegant description of electrons in graphene as particles endowed with a pseudospin degree-of-freedom.1 At low energy, electrons in graphene are described by a 2D massless Dirac fermion ͑MDF͒ Hamiltonian, H D = v F · p, where v F is the bare Fermi velocity, which does not depend on carrier density, p is the 2D momentum measured from the corners of the Brillouin zone, and is the pseudospin operator constructed with two Pauli matrices ͕ i , i = x , y͖, which act on the sublattice pseudospin degree-of-freedom. It follows that the energy eigenstates are chiral, i.e., for a given p have pseudospins oriented either parallel ͑conduction band͒ or antiparallel ͑valence band͒ to p. The Dirac-like wave equation and the chirality of its eigenstates have a number of very intriguing implications.1 It would be highly desirable to have other materials with Dirac-like spectrum and a pseudospin degree-offreedom. One candidate is represented by HgTe/Hg͑Cd͒Te quantum wells ͑QWs͒ where MDFs are predicted to arise at a critical QW thickness.2 More recently, Park and Louie 3 proposed that MDFs can arise in any 2D electron gas ͑2DEG͒ if appropriately nanopatterned.Here we present an independent approach to the realization of "artificial graphene" in a nanopatterned 2DEG. We provide theoretical evidence for the occurrence of linearly dispersing energy bands in an artificially engineered honeycomb lattice, and we demonstrate a remarkable dependence of the Fermi velocity on the strength of the external potential in this system. We also define the conditions that the external periodic potential and the electron density must satisfy in order to achieve artificial MDFs. Finally we present the photoluminescence ͑PL͒ of the 2DEG confined in a highmobility modulation-doped GaAs/AlGaAs QW where a nanopatterning with honeycomb symmetry is achieved by dry etching. We believe that the development of patterned 2DEGs with tunable parameters will offer unprecedented opportunities to study fundamental interactions of MDFs in high-mobility semiconductor structures.We start our analysis by considering a 2DEG consisting of electrons with band mass m b = 0.067m ͑m is the bare electron mass in vacuum͒ confined in a GaAs/AlGaAs QW. The 2DEG is subjected to a...

Artificial crystal lattices can be used to tune repulsive Coulomb interactions between electrons. We trapped electrons, confined as a two-dimensional gas in a gallium arsenide quantum well, in a nanofabricated lattice with honeycomb geometry. We probed the excitation spectrum in a magnetic field, identifying collective modes that emerged from the Coulomb interaction in the artificial lattice, as predicted by the Mott-Hubbard model. These observations allow us to determine the Hubbard gap and suggest the existence of a Coulomb-driven ground state.

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1−xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour–liquid–solid growth. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 °C) during cool-down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth.

Stable metal nanoclusters (NCs) with uniform interior nanogaps reproducibly offer a highly robust substrate for surface-enhanced Raman scattering (SERS) because of the presence of abundant hot spots on their surface. The synthesis of such an SERS substrate by a simple route is a challenging task. Here, we have synthesized a highly stable wirelike cluster of silver nanoparticles (Ag-NPs) with an interparticle gap of ~1.7 ± 0.2 nm using deoxyribonucleic acid (DNA) as the template by exploiting an easy and inexpensive chemical route. The red shift in the surface plasmon resonance (SPR) band of Ag-NCs compared to SPR of a single Ag-NP confirms the strong interplasmonic interaction. Methylene Blue (MB) is used as a representative Raman probe to study the SERS effect of the NCs. The SERS measurements reveal that uniform, reproducible, and strong Raman signals were observed up to the single-molecule level. The intensity of the Raman signal is not highly dependent on the polarization of the excitation laser. The DNA-based Ag-NCs as a substrate show better isotropic behavior for their SERS intensity compared to the dimer, as confirmed from both the experimental and theoretical simulation results. We believe that in the future the DNA-based Ag-NCs might be useful as a potential SERS substrate for ultrasensitive trace detection, biomolecular assays, NP-based photothermal therapeutics, and a few other technologically important fields.