Strong Charge-Transfer Doping of 1 to 10 Layer Graphene by NO₂

We use resonance Raman and optical reflection contrast methods to study charge transfer in 1–10 layer (1L–10L) thick graphene samples on which NO₂ has adsorbed. Electrons transfer from the graphene to NO₂, leaving the graphene layers doped with mobile delocalized holes. Doping follows a Langmuir-type isotherm as a function of NO₂ pressure. Raman and optical contrast spectra provide independent, self-consistent measures of the hole density and distribution as a function of the number of layers (<i>N</i>). At high doping, as the Fermi level shift <i>E</i>_F reaches half the laser photon energy, a resonance in the graphene G mode Raman intensity is observed. We observe a decrease of graphene optical absorption in the near-IR that is due to hole-doping. Highly doped graphene is more optically transparent and much more electrically conductive than intrinsic graphene. In thicker samples, holes are effectively confined near the surface, and in these samples, a small band gap opens near the surface. We discuss the properties and versatility of these highly charge-transfer-doped, few-layer-thick graphene samples as a new class of electronic materials

The Role of Photon Energy and Semiconductor Substrate in the Plasmon-Mediated Photooxidation of Citrate by Silver Nanoparticles

The plasmon-mediated photooxidation of citrate ions adsorbed on silver (Ag) nanoparticle–semiconductor electrodes is studied in a photoelectrochemical cell. Consistent with previous reports, a negative photovoltage and an anodic photocurrent arise from citrate photooxidation under weak visible light illumination. We measure the wavelength dependence of this reaction for three different types of Ag nanoparticles and find that both the photovoltage and photocurrent increase with photon energy over the visible spectral range. The electrode photoresponse does not closely track the localized surface plasmon resonance of the Ag nanoparticles. We also explore the role of the semiconductor substrate in this reaction, and we find a similar electrode photoresponse for several different substrates. The strong dependence of reaction rate on photon energy is consistent with a hot-carrier photochemical process where photoexcited hot holes generated in the Ag nanoparticles are responsible for the oxidation of adsorbed citrate

Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS₂ Crystals

We have investigated the phase transformation of bulk MoS₂ crystals from the metastable metallic 1T/1T′ phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T′ material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with <i>in situ</i> Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T′ to 2H in a single laser pass. The mechanisms for the phase transformation are discussed

Observation of Excitonic Rydberg States in Monolayer MoS₂ and WS₂ by Photoluminescence Excitation Spectroscopy

We have identified excited exciton states in monolayers of MoS₂ and WS₂ supported on fused silica by means of photoluminescence excitation spectroscopy. In monolayer WS₂, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS₂, excited exciton transitions are observed at energies of 2.24 and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV

Physical Adsorption and Charge Transfer of Molecular Br₂ on Graphene

We present a detailed study of gaseous Br₂ adsorption and charge transfer on graphene, combining <i>in situ</i> Raman spectroscopy and density functional theory (DFT). When graphene is encapsulated by hexagonal boron nitride (h-BN) layers on both sides, in a h-BN/graphene/h-BN sandwich structure, it is protected from doping by strongly oxidizing Br₂. Graphene supported on only one side by h-BN shows strong hole doping by adsorbed Br₂. Using Raman spectroscopy, we determine the graphene charge density as a function of pressure. DFT calculations reveal the variation in charge transfer per adsorbed molecule as a function of coverage. The molecular adsorption isotherm (coverage <i>versus</i> pressure) is obtained by combining Raman spectra with DFT calculations. The Fowler–Guggenheim isotherm fits better than the Langmuir isotherm. The fitting yields the adsorption equilibrium constant (∼0.31 Torr^–1) and repulsive lateral interaction (∼20 meV) between adsorbed Br₂ molecules. The Br₂ molecule binding energy is ∼0.35 eV. We estimate that at monolayer coverage each Br₂ molecule accepts 0.09 e^– from single-layer graphene. If graphene is supported on SiO₂ instead of h-BN, a threshold pressure is observed for diffusion of Br₂ along the (somewhat rough) SiO₂/graphene interface. At high pressure, graphene supported on SiO₂ is doped by adsorbed Br₂ on both sides

Controlled Electrochemical Intercalation of Graphene/<i>h-</i>BN van der Waals Heterostructures

Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electrochemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (<i>h</i>-BN). We demonstrate that encapsulating graphene with <i>h</i>-BN eliminates parasitic surface side reactions while simultaneously creating a new heterointerface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density >5 × 10¹³ cm² with mobility >10³ cm²/(V s) in the most heavily intercalated samples, where Shubnikov–de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures

Energy Transfer from Quantum Dots to Graphene and MoS₂: The Role of Absorption and Screening in Two-Dimensional Materials

We report efficient nonradiative energy transfer (NRET) from core–shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS₂ of single- and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1–10 ns^–1. Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS₂ layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials

Ferromagnetic Ordering in Superatomic Solids

In order to realize significant benefits from the assembly of solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes undergo a ferromagnetic phase transition at low temperatures. Moreover, we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways