David Perello

Transferred wrinkled Al2O3 for highly stretchable and transparent graphene–carbon nanotube transistors

Despite recent progress in producing transparent and bendable thin-film transistors using graphene and carbon nanotubes, the development of stretchable devices remains limited either by fragile inorganic oxides or polymer dielectrics with high leakage current. Here we report the fabrication of highly stretchable and transparent field-effect transistors combining graphene/single-walled carbon nanotube (SWCNT) electrodes and a SWCNT-network channel with a geometrically wrinkled inorganic dielectric layer. The wrinkled Al2O3 layer contained effective built-in air gaps with a small gate leakage current of 10(-13) A. The resulting devices exhibited an excellent on/off ratio of ~10(5), a high mobility of ~40 cm(2) V(-1) s(-1) and a low operating voltage of less than 1 V. Importantly, because of the wrinkled dielectric layer, the transistors retained performance under strains as high as 20% without appreciable leakage current increases or physical degradation. No significant performance loss was observed after stretching and releasing the devices for over 1,000 times. The sustainability and performance advances demonstrated here are promising for the adoption of stretchable electronics in a wide variety of future applications.

Small hysteresis nanocarbon-based integrated circuits on flexible and transparent plastic substrate.

We report small hysteresis integrated circuits by introducing monolayer graphene for the electrodes and a single-walled carbon nanotube network for the channel. Small hysteresis of the device originates from a defect-free graphene surface, where hysteresis was modulated by oxidation. This uniquely combined nanocarbon material device with transparent and flexible properties shows remarkable device performance; subthreshold voltage of 220 mV decade(-1), operation voltage of less than 5 V, on/off ratio of approximately 10(4), mobility of 81 cm(2) V(-1) s(-1), transparency of 83.8% including substrate, no significant transconductance changes in 1000 times of bending test, and only 36% resistance decrease at a tensile strain of 50%. Furthermore, because of the nearly Ohmic contact nature between the graphene and carbon nanotubes, this device demonstrated a contact resistance 100 times lower and a mobility 20 times higher, when compared to an Au electrode.

High-performance n-type black phosphorus transistors with type control via thickness and contact-metal engineering

Recent work has demonstrated excellent p-type field-effect switching in exfoliated black phosphorus, but type control has remained elusive. Here, we report unipolar n-type black phosphorus transistors with switching polarity control via contact-metal engineering and flake thickness, combined with oxygen and moisture-free fabrication. With aluminium contacts to black phosphorus, a unipolar to ambipolar transition occurs as flake thickness increases from 3 to 13 nm. The 13-nm aluminium-contacted flake displays graphene-like symmetric hole and electron mobilities up to 950 cm2 V−1 s−1 at 300 K, while a 3 nm flake displays unipolar n-type switching with on/off ratios greater than 105 (107) and electron mobility of 275 (630) cm2 V−1 s−1 at 300 K (80 K). For palladium contacts, p-type behaviour dominates in thick flakes, while 2.5–7 nm flakes have symmetric ambipolar transport. These results demonstrate a leap in n-type performance and exemplify the logical switching capabilities of black phosphorus.

Oxidation Effect in Octahedral Hafnium Disulfide Thin Film

Atomically smooth van der Waals materials are structurally stable in a monolayer and a few layers but are susceptible to oxygen-rich environments. In particular, recently emerging materials such as black phosphorus and perovskite have revealed stronger environmental sensitivity than other two-dimensional layered materials, often obscuring the interesting intrinsic electronic and optical properties. Unleashing the true potential of these materials requires oxidation-free sample preparation that protects thin flakes from air exposure. Here, we fabricated few-layer hafnium disulfide (HfS2) field effect transistors (FETs) using an integrated vacuum cluster system and study their electronic properties and stability under ambient conditions. By performing all the device fabrication and characterization procedure under an oxygen- and moisture-free environment, we found that few-layer AA-stacking HfS2-FETs display excellent field effect responses (Ion/Ioff ≈ 10(7)) with reduced hysteresis compared to the FETs prepared under ambient conditions. Oxidation of HfS2 occurs uniformly over the entire area, increasing the film thickness by 250% at a prolonged oxidation time of >120 h, while defects on the surface are the preferential initial oxidation sites. We further demonstrated that the stability of the device in air is significantly improved by passivating FETs with BN in a vacuum cluster.

Synthesis of Edge-Closed Graphene Ribbons with Enhanced Conductivity

Edge-closed and edge-opened graphene ribbons were synthesized on Pd nanowire templates using plasma-enhanced chemical vapor deposition (PECVD). After metal nanowire etching, the tubular shaped thin graphene layers were collapsed to edge-closed graphene ribbon. In order to make edge-opened graphene ribbons, the graphene layers on the top part of the metal nanowire were selectively etched by O(2) plasma. The protected graphene layers at the bottom of nanowire are transformed to edge-opened graphene ribbon after nanowire etching. Because of defect-free edges, edge-closed graphene ribbon showed reduced D-band intensity compared to edge-opened graphene ribbons, and moreover, the conductivity of edge-closed graphene ribbon was much higher than that of edge-opened graphene ribbon.

Stranski–Krastanov and Volmer–Weber CVD Growth Regimes To Control the Stacking Order in Bilayer Graphene

Aside from unusual properties of monolayer graphene, bilayer has been shown to have even more interesting physics, in particular allowing bandgap opening with dual gating for proper interlayer symmetry. Such properties, promising for device applications, ignited significant interest in understanding and controlling the growth of bilayer graphene. Here we systematically investigate a broad set of flow rates and relative gas ratio of CH4 to H2 in atmospheric pressure chemical vapor deposition of multilayered graphene. Two very different growth windows are identified. For relatively high CH4 to H2 ratios, graphene growth is relatively rapid with an initial first full layer forming in seconds upon which new graphene flakes nucleate then grow on top of the first layer. The stacking of these flakes versus the initial graphene layer is mostly turbostratic. This growth mode can be likened to Stranski-Krastanov growth. With relatively low CH4 to H2 ratios, growth rates are reduced due to a lower carbon supply rate. In addition bi-, tri-, and few-layer flakes form directly over the Cu substrate as individual islands. Etching studies show that in this growth mode subsequent layers form beneath the first layer presumably through carbon radical intercalation. This growth mode is similar to that found with Volmer-Weber growth and was shown to produce highly oriented AB-stacked materials. These systematic studies provide new insight into bilayer graphene formation and define the synthetic range where gapped bilayer graphene can be reliably produced.

Anomalous schottky barriers and contact band-to-band tunneling in carbon nanotube transistors

Devices incorporating nanoscale materials, particularly carbon nanotubes (CNTs), offer exceptional electrical performance. Absent, however, is an experimentally backed model explaining contact-metal work function, device layout, and environment effects. To fill the void, this report introduces a surface-inversion channel model based on low temperature and electrical measurements of a distinct single-walled semiconducting CNT contacted by Hf, Cr, Ti, and Pd electrodes. Anomalous barrier heights and metal-contact dependent band-to-band tunneling phenomena are utilized to show that, dependent upon contact work function and gate field, transport occurs either directly between the metal and CNT channel or indirectly via injection of carriers from the metal-covered CNT region to the CNT channel. The model is consistent with previously contradictory experimental results, and the methodology is simple enough to apply in other contact-dominant systems.

Analysis of hopping conduction in semiconducting and metallic carbon nanotube devices

Single-walled carbon nanotube field-effect transistors were irradiated with 20 keV electrons using an e-beam lithography exposure method. Analysis of conduction data in the temperature range from 25 to 300 K indicated the creation of insulating regions containing traps along the nanotube channel. Further analysis of semiconducting and metallic nanotube devices shows dramatic differences in the effect of the electron exposure on the hopping defect barrier heights. Barriers for metallic nanotubes saturate at significantly larger values than semiconducting nanotubes due to shorter localization lengths. The limited and near constant density of states at the Fermi level induces a larger hopping length to localization length ratio, further limiting current and increasing measured trap heights. Poole–Frenkel hopping with an adjustment for electron localization is utilized to explain the inconsistencies. n-type and p-type barriers in the nanotube devices displayed exponential dependence on applied gate voltage bi...

Photocurrent of CdSe nanocrystals on single-walled carbon nanotube-field effect transistor

CdSe nanocrystals (NCs) have been decorated on single-walled carbon nanotubes (SWCNTs) by combining a method of chemically modified substrate along with gate-bias control. CdSe∕ZnS core/shell quantum dots were negatively charged by adding mercaptoacetic acid. The silicon oxide substrate was decorated by octadecyltrichlorosilane and converted to hydrophobic surface. The negatively charged CdSe NCs were adsorbed on the SWCNT surface by applying a negative gate bias. The measured photocurrent clearly demonstrates that CdSe NCs decorated SWCNT can be used for photodetector and solar cell that are operable over a wide range of wavelengths.

A Single Palladium Nanowire Via Electrophoresis Deposition Used as a Ultrasensitive Hydrogen Sensor

We have successfully fabricated and demonstrated the use of a single metallic nanowire as a hydrogen sensor with extremely high sensitivity via a very simple fabrication process. In this paper, single palladium (Pd) nanowires were electrodeposited within 100-nm-wide polymethylmethacrylate nanochannels using a Pd electrolyte solution. Via this method, nanowires were grown with widths ranging from 50 to 100 nm, and lengths from 3 to 7 mum. The nanowires were successfully used to sense hydrogen concentrations as low as 5 ppm at room temperature. The growth control of single Pd nanowires, as well as the primary sensing mechanism, is addressed in detail in this paper.