Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF(2)) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C(4)F), which is optically transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C(4)F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C(4)F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphene's carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF(2) gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.
We report on the transport properties of random networks of single-wall carbon nanotubes fabricated into thin-film transistors. At low nanotube densities ͑ϳ1 m Ϫ2 ͒ the networks are electrically continuous and behave like a p-type semiconductor with a field-effect mobility of ϳ10 cm 2 /V s and a transistor on-to-off ratio ϳ10 5 . At higher densities ͑ϳ10 m Ϫ2 ͒ the field-effect mobility can exceed 100 cm 2 /V s; however, in this case the network behaves like a narrow band gap semiconductor with a high off-state current. The fact that useful device properties are achieved without precision assembly of the nanotubes suggests the random carbon nanotube networks may be a viable material for thin-film transistor applications.Perhaps the most intriguing electronic property of single-wall carbon nanotubes ͑SWNTs͒ is the high roomtemperature mobility of semiconducting SWNTs ͑s-SWNTs͒ that is more than an order of magnitude larger than the mobility of crystalline Si. 1,2 This high mobility has prompted researchers to fabricate and study field-effect transistors in which a single s-SWNT serves as a high-mobility transport channel. [1][2][3][4][5][6][7] Recent measurements on such devices yield a transconductance per unit channel width greater than that of state-of-the-art Si transistors. 7 However, because of the limited current-carrying capacity of individual SWNTs, many s-SWNTs aligned side by side in a single device would be required in order to surpass the current drive of a Si device. Such precise positioning of SWNTs is beyond the capability of current growth and assembly technology and presents a major technological hurdle for carbon nanotube-based electronic applications.In contrast, random arrays of SWNTs are easily produced either by direct growth on a catalyzed substrate or by deposition onto an arbitrary substrate from a solution of suspended SWNTs. If the density of SWNTs in such an array is sufficiently high, the nanotubes will interconnect and form continuous electrical paths. Such random arrays of SWNTs have not previously been seriously investigated for use as channels in field-effect transistors.In this letter we explore the transport properties of random networks of SWNTs and find that low density networks ͑ϳ1 m Ϫ2 ͒ behave like a p-type semiconducting thin film with a field-effect mobility ϳ10 cm 2 /V s, approximately an order of magnitude larger than the mobility of materials typically used in commercial thin-film transistors, e.g., amorphous Si. These mobility values and correspondingly good electronic quality of the random SWNT network are due to a combination of the low resistance of inter-SWNT contacts and the high mobility of the individual SWNTs, which together compensate for the extremely low fill factor of the network. These initial transport results are promising and indicate that such random nanotube networks ͑easily produced with no need for precision assembly͒ form an interest-ing electronic material that has potential for use in thin-filmtransistor applications to produce active electronic...
We report a photoluminescence study of excitons localized by interface fluctuations in a narrow GaAs͞AlGaAs quantum well. This type of structure provides a valuable system for the optical study of quantum dots. By reducing the area of the sample studied down to the optical near-field regime, only a few dots are probed. With resonant excitation we measure the excited-state spectra of single quantum dots. Many of the spectral lines are linearly polarized with a fine structure splitting of 20 -50 meV. These optical properties are consistent with the characteristic asymmetry of the interface fluctuations. PACS numbers: 78.55.Cr, 71.35.Cc In this Letter we describe the polarization dependence of the optical spectra of single naturally formed GaAs quantum dots. Most previous optical studies of quantum dots (QDs) have probed large ensembles which have led to inhomogeneous broadening of the spectral features. However, recently several groups have shown that it is possible to study single QDs with photoluminescence (PL) either by reducing the size of the sample, [1] by cathodoluminescence [2,3], or by reducing the size of the laser spot on the sample through microscopic [4,5] or optical near-field techniques [6]. Here we use a similar technique whereby we combine high spatial and spectral resolution optics with excitation spectroscopy to study in detail the spectrum of a single QD [7]. With improved resolution we are able to resolve the spectral lines and to study the polarization dependence of the PL spectrum of an individual QD. We often find that the PL is linearly polarized along the (110) crystal axes and observe a fine structure splitting in each of the spectral lines. These results are analogous to the early days of atomic spectroscopy as improvements in techniques allowed the observation of fine structure splittings in the optical spectra. However, the physical phenomena responsible for the effects presented here are unique to the quantized condensed matter system.The QDs we have studied were formed naturally by interface steps in narrow quantum wells [4][5][6][7]. Specifically, the electrons and holes become localized into QDs in regions of the quantum well that are a monolayer wider than the surrounding region and, therefore, have a slightly smaller confinement energy. These well width fluctuations arise from monolayer-high islands at the interfaces which are randomly formed on the growth-interrupted surface by the migration of the cations to step edges. By interrupting the growth these islands can grow to diameters larger than the exciton Bohr diameter (20 nm). A scanning tunneling microscope image of a growth-interrupted GaAs surface grown under similar conditions as our quantum dot sample is shown in Fig. 1. Large monolayer-high islands of varying lateral sizes are evident, and the islands tend to be elongated along the [110] crystal axis. Thus we intuitively expect that the optical properties associated with the localized excitons will reflect this characteristic interface structure. In fact, as we will...
We show that the capacitance of single-walled carbon nanotubes (SWNTs) is highly sensitive to a broad class of chemical vapors and that this transduction mechanism can form the basis for a fast, low-power sorption-based chemical sensor. In the presence of a dilute chemical vapor, molecular adsorbates are polarized by the fringing electric fields radiating from the surface of a SWNT electrode, which causes an increase in its capacitance. We use this effect to construct a high-performance chemical sensor by thinly coating the SWNTs with chemoselective materials that provide a large, class-specific gain to the capacitance response. Such SWNT chemicapacitors are fast, highly sensitive, and completely reversible.
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