The evolution of grapheneʼs electrical transport properties due to processing with the polymer polymethyl methacrylate (PMMA) and heat are examined in this study. The use of stencil (shadow mask) lithography enables fabrication of graphene devices without the usage of polymers, chemicals or heat, allowing us to measure the evolution of the electrical transport properties during individual processing steps from the initial as-exfoliated to the PMMA-processed graphene. Heating generally promotes the conformation of graphene to SiO2 and is found to play a major role for the electrical properties of graphene while PMMA residues are found to be surprisingly benign. In accordance with this picture, graphene devices with initially high carrier mobility tend to suffer a decrease in carrier mobility, while in contrast an improvement is observed for low carrier mobility devices. We explain this by noting that flakes conforming poorly to the substrate will have a higher carrier mobility which will however be reduced as heat treatment enhance the conformation. We finally show the electrical properties of graphene to be reversible upon heat treatments in air up to 200 °C.

Chemical vapor deposited graphene is nanopatterned by a spherical block-copolymer etch mask. The use of spherical rather than cylindrical block copolymers allows homogeneous patterning of cm-scale areas without any substrate surface treatment. Raman spectroscopy was used to study the controlled generation of point defects in the graphene lattice with increasing etching time, confirming that alongside the nanomesh patterning, the nanopatterned CVD graphene presents a high defect density between the mesh holes. The nanopatterned samples showed sensitivities for NO2 of more than one order of magnitude higher than for non-patterned graphene. NO2 concentrations as low as 300 ppt were detected with an ultimate detection limit of tens of ppt. This is so far the smallest value reported for not UV illuminated graphene chemiresistive NO2 gas sensors. The drastic improvement in the gas sensitivity is believed to be due to the high adsorption site density, thanks to the combination of edge sites and point defect sites. This work opens the possibility of large area fabrication of nanopatterned graphene with extreme density of adsorption sites for sensing applications.

Block copolymer self-assembly holds great promise as a rapid, cheap and scalable approach to nanolithography. We present a straightforward method for fabrication of sub-10 nm line patterns from a lamellar polystyrene-b-polydimethylsiloxane (SD) block copolymer with total average molecular weight of 10.5 kg/mol. Thin SD films directly spin cast onto silicon substrates and on graphene, form regular line patterns of sub-10 nm pitch on the substrates after few minutes of annealing at 45 ºC in the presence of toluene vapour. Perfect pattern alignment was achieved by confining the films inside the trenches of graphoepitaxial substrates. The SD template was furthermore used as lithographic mask to fabricate high-quality sub-10 nm graphene nanoribbons.This was realized by one step oxygen plasma treatment, which accomplishes three tasks: hardening the PDMS block by oxidation, and etching both the PS block and the graphene under PS. Raman analysis supports the formation of graphene nanoribbons with an average distance between defects corresponding to the oxidized PDMS pitch, with no sign of defects generated in the ribbon channel.This suggests a high degree of protection of the nanoribbons by the hard oxidized PDMS mask formed in situ during oxygen plasma etching.

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In the framework of the development of an ultrasensitive microfabricated mass sensor for distributed mass sensing applications we present a bulk resonator-based mass sensor. The two devices presented are based on a polysilicon disk resonating at 132 and 66 MHz, respectively, actuated electrostatically in a wine-glass mode. By using bulk mode resonators it has been possible to reduce the thickness of the sensor layer without affecting the resonance frequency, reaching an extremely high distributed mass to a frequency shift sensitivity of 11.3 kHz µm2 fg−1 and a markedly small mass resolution of 8.7 pg cm−2 in air and at room temperature.

There is a fundamental need for techniques for thin film characterization. The current options for obtaining infrared (IR) spectra typically suffer from low signal-to-noise-ratios (SNRs) for sample thicknesses confined to a few nanometers. We present nanomechanical infrared spectroscopy (NAM-IR), which enables the measurement of a complete infrared fingerprint of a polyvinylpyrrolidone (PVP) layer as thin as 20 nm with an SNR of 307. Based on the characterization of the given NAM-IR setup, a minimum film thickness of only 160 pm of PVP can be analyzed with an SNR of 2. Compared to a conventional attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) system, NAM-IR yields an SNR that is 43 times larger for a 20 nm-thick PVP layer and requires only a fraction of the acquisition time. These results pave the way for NAM-IR as a highly sensitive, fast, and practical tool for IR analysis of polymer thin films.

Abstract:We investigate graphene that has been patterned with a short nanomesh -a small number of rows of antidots perpendicular to the current flow. Theoretical reports have suggested that a short antidot lattice in graphene can generate an energy gap with a relatively small reduction of the transmission compared to what is typically associated with nanoribbon and nanomesh devices. Exfoliated graphene flakes were electrically contacted allowing for four-terminal electrical measurements. Antidot lattices were then defined using 100 keV electron beam lithography. Electrical measurements showed that a few rows (1 or 5) had comparable mobilities (>100 cm 2 /Vs), while a large number of rows, around 40, led to a strong reduction of apparent carrier mobility (<5 cm 2 /Vs). The carrier mobility was measured as a function of temperature, with the low-temperature behaviour being well described by variable range hopping. This work produced the highest pattern density (30 nm hole diameter and neckwidth) reported for graphene using electron beam lithography.

We have fabricated an ultrasensitive nanomechanical resonator based on the extensional vibration mode to weigh the adsorbed water on self-assembled monolayers of DNA as a function of the relative humidity. The water adsorption isotherms provide the number of adsorbed water molecules per nucleotide for monolayers of single stranded (ss) DNA and after hybridization with the complementary DNA strand. Our results differ from previous data obtained with bulk samples, showing the genuine behavior of these self-assembled monolayers. The hybridization cannot be inferred from the water adsorption isotherms due to the low hybridization efficiency of these highly packed monolayers. Strikingly, we efficiently detect the hybridization by measuring the thermal desorption of water at constant relativity humidity. This finding adds a new nanomechanical tool for developing a label-free nucleic acid sensor based on the interaction between water and self-assembled monolayers of nucleic acids.