Chemical functionalization is a promising route to band gap engineering of graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer graphene sheets in the form of substrate-supported or free-standing films. Our transport measurements demonstrate that nonsuspended functionalized graphene behaves as a granular metal, with variable range hopping transport and a mobility gap ~ 0.1 eV at low temperature. For suspended graphene that allows functionalization on both surfaces, we demonstrate tuning of its electronic properties from a granular metal to a gapped semiconductor, in which charge transport occurs via thermal activation over a gap ~ 80 meV. This non-invasive and scalable functionalization technique paves the way for CMOS-compatible band gap engineering of graphene electronic devices.

Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane

Pristine single layer graphene (SLG) has exceedingly high mobility, which is ∼ 4,000-20,000 cm 2 /Vs for typical devices supported on Si/SiO 2 substrates, and may reach as high as 250,000 cm 2 /Vs in suspended devices at room temperature. [ 1 ] Such high mobilities make graphene an extremely attractive candidate for the next generation electronic materials. However, the absence of a band gap, which is necessary for digital electronics, presents a technological challenge. One effective approach to band gap engineering is the (partial) saturation of the valences of some of the conjugated carbon atoms. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Nitrophenyl functionalization, in which a fully rehybridized sp 3 carbon atom is created in the lattice, dramatically modifi es the electronic and magnetic structure of graphene, with signifi cantly reduced fi eld effect mobility. [18][19][20][21][22] Since this type of functionalization scheme introduces resonant scatters [ 23 ] into the graphene lattice, we refer to this as destructive rehybridization. [ 24 ] Most approaches for chemical modifi cation of graphene involve the creation of sp 3 carbon centers at the cost of conjugated sp 2 carbon atoms in the graphene lattice. We have recently investigated the application of organometallic chemistry by studying the covalent hexahapto modifi cation of graphitic surfaces with zero-valent transition metals such as chromium. [ 12 , 25 ] The formation of the hexahapto ( η 6 )-arene − metal bond leads to very little structural reorganization of the π -system. In the reaction of the zero-valent chromium metal with graphene, the vacant d π orbital of the metal (chromium) constructively overlaps with the occupied π -orbitals of graphene, without removing any of the sp 2 carbon atoms from conjugation. [ 12 , 25 ] Previously we have shown that the formation of such bishexahapto transition metal bonds between the conjugated surfaces of the benzenoid ring systems present in the surfaces of graphene and carbon nanotubes can dramatically change their electrical properties. [ 12 , 24-27 ] These prior works focus on using the bis-hexahapto-metal bond as an interconnect for electrical transport between the conjugated surfaces, thereby increasing the dimensionality of the carbon nanotube and graphene materials and thus we were concerned with the use of the bishexahapto-metal bond as a conduit for electron transport between surfaces. In contrast, the goal of the present study is to investigate the effect of the hexahapto-bonded chromium atoms on the electronic properties of graphene itself (within the plane of a single layer), by using mono-hexahapto-metal bonds to the graphene surface.Single layer graphene (SLG) fl akes used in this study were extracted from bulk graphite using a standard mechanical exfoliation method and placed on a Si substrate with 300 nm SiO 2 . Contacts consisting of 10 nm of Cr and 150 nm of Au were deposited on SLG by e-beam lithography. The devices were then annealed in vacuum by passing a high current...

Thermal conductivity measurements over variable lengths on nanostructures such as nanowires provide important information about the mean free paths (MFPs) of the phonons responsible for heat conduction. However, nearly all of these measurements have been interpreted using an average MFP even though phonons in many crystals possess a broad MFP spectrum. Here, we present a reconstruction method to obtain MFP spectra of nanostructures from variable-length thermal conductivity measurements. Using this method, we investigate recently reported length-dependent thermal conductivity measurements on SiGe alloy nanowires and suspended graphene ribbons. We find that the recent measurements on graphene imply that 70% of the heat in graphene is carried by phonons with MFPs longer than 1 micron.

Narrow gaps are formed in suspended single- to few-layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with voltages of 2.5 to ~4.5 V, corresponding to an ON pulse, and ~8 V, corresponding to an OFF pulse. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement and underscores the potential of all-carbon devices for integration with graphene electronics.

Which sizes of nanoparticles embedded in a crystalline solid yield the lowest thermal conductivity? Nanoparticles have long been demonstrated to reduce the thermal conductivity of crystals by scattering phonons, but most previous works assumed the nanoparticles to have a single size. Here, we use optimization methods to show that the best nanoparticle size distribution to scatter the broad thermal phonon spectrum is not a similarly broad distribution but rather several discrete peaks at well-chosen nanoparticle radii. For SiGe, the best size distribution yields a thermal conductivity below that of amorphous silicon. Further, we demonstrate that a simplified distribution yields nearly the same low thermal conductivity and can be readily fabricated. Our work provides important insights into how to manipulate the full spectrum of phonons and will guide the design of more efficient thermoelectric materials.

The reflection of waves from rough surfaces is a fundamental process that plays a role in diverse fields such as optics, acoustics, and seismology. While a quantitative understanding of the reflection process has long been established for many types of waves, the precise manner in which thermal phonons of specific wavelengths reflect from atomically rough surfaces remains unclear owing to limited control over terahertzfrequency phonon generation and detection. Knowledge of these processes is critical for many applications, however, and is particularly important for recent attempts to create novel materials by coherently interfering thermal phonons. Here, we report measurements of a key property for these efforts, the phononwavelength-dependent specularity parameter, which describes the probability of specular reflections of thermal phonons at a surface. Our experiments show evidence of specular surface reflections of terahertz thermal phonons in our samples around room temperature and indicate a sensitivity of these reflections to surface imperfections on the scale of just 2-3 atomic planes. Our work demonstrates a general route to probe the microscopic interactions of thermal phonons with surfaces that are typically inaccessible with traditional experiments.

Common glass is absorbing in the mid-infrared but transparent to sunlight, keeping our greenhouses and rooms warm. But a visibly-transparent and infrared-reflective material will perform much better than glass. Engineered multilayer optical coatings have been able to achieve both visible transparency and infrared reflectivity, but manufacturing cost has prevented their use on a large scale. Here, we predicted and successfully synthesized a transparent wavelength-selective metal-polymer hybrid films with low emissivity of less than 0.1 in the infrared range. The films, based on silver nanowires and PMMA, exhibit high transmission (> 85%) through the visible wavelength range and high reflectance (> 90%) in the mid-wavelength and long-wavelength infrared range. Our films are more transparent than a commercially available multilayer engineered coating in the visible and are much easier to fabricate. On an average sunny day, our films in this work warm up a prototype greenhouse 8 degrees Celsius higher than that of glass. We believe that our films hold promise for large scale applications, leading to significant energy savings for indoor heating.