Dry Transfer Method Research Articles

Fabrication of zigzag-edged graphene antidot lattice and its transport properties

Graphene nanostructures with defined edges are proposed as a promising platform for the realization of nano-electronics and spin-electronics. However, patterned graphene nanostructure can lead to extra damage and drastically reduce its charge carrier mobility due to the edge disorder. The high flexibility of a top-down patterning method with edge smoothness is extremely desirable. Hydrogen plasma enhanced anisotropic etching graphene is demonstrated to be an efficient method of fabricating zigzag-edge graphene nanostructures. In addition, boron nitride is shown to be an excellent substrate for graphene due to its atomic flatness. Here in this work, we fabricate zigzag edge graphene antidot lattices on a boron nitride substrate via dry transfer method and traditional electron beam lithography, and reactive ion etching followed by hydrogen anisotropic etching approach. At low magnetic fields, weak localization is observed and its visibility is enhanced by intervalley scattering on antidot edges. We observe commensurate features in magnetotransport properties which stem from carriers around one antidot, signifying the high quality of our patterned samples. At high magnetic field, crossover from Shubnikov-de Haas oscillation to quantum Hall effect can be clearly observed due to the high mobility of our zigzag edge graphene antidot lattices. The transport properties of our patterned samples suggest that our fabrication method paves the way for achieving high quality graphene antidot lattices. High quality zigzag edge graphene antidot lattice might be a great platform to study the transport properties of lateral superlattice potential modulation graphene.

CVD graphene transfer procedure to the surface of stainless steel for stem cell proliferation

Graphene is an attractive material for tissue engineering due to its good interaction with cells (adhesion and proliferation) and its ability to protect the surface of biomaterials against corrosion. CVD graphene is a suitable candidate among the various types of graphene since it can cover large areas, however, the transfer process to the target substrates still involves several steps including the etching of metal used in chemical vapor deposition. Here, a new route of CVD graphene transfer process to the surface of austenitic stainless steel (ASS) is presented taking the advantage of not requiring an etching step. The structural stability of graphene transferred to the ASS surface was evaluated in SBF (simulated blood fluid) as well as for stem cell attachment and proliferation. CVD graphene was successfully transferred to a stainless steel surface in two steps: 1) Directly from a polymer film without the etching of copper (Direct Dry Transfer method - DDT); 2) Graphene transfer from the polymer to the ASS surface using temperature and pressure and the removal of the polymer by solvent. The area covered by graphene on the ASS surface varied according to the transfer parameters (pressure and time) and roughness of the substrate. Raman mapping results showed that graphene transferred to ASS surfaces presents a distinct structural stability in SBF of up to 28days. All samples tested allowed mesenchymal stem cell attachment and their proliferation. The success of the transfer procedure, the structural stability presented by the graphene under ASS, its attachment capacity, and proliferation of cells provides new opportunities for graphene as a covering material for tissue engineering.

Superior thermal conductivity in suspended bilayer hexagonal boron nitride.

We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temperature thermal conductivity is around 484 Wm−1K−1(+141 Wm−1K−1/ −24 Wm−1K−1) which exceeds that in bulk h-BN, providing experimental observation of the thickness-dependent thermal conductivity in suspended few-layer h-BN.

Green production of pristine graphene using fluid dynamic force in supercritical CO2

A green and scalable method was presented for the production of pristine graphene by the exfoliation of graphite using a shear mixer in supercritical CO2. The resulting graphene flakes were characterized by transmission electron microscopy, Raman spectroscopy and atomic force microscopy. The synergistic effects of the fluid dynamic force coupled with the supercritical CO2 actualized the exfoliation of graphite into graphene. The total exfoliation yield reached 63.2%. Up to 79% of the produced graphene was less than five layers, of which monolayer, bilayer and trilayer represented 27%, 25% and 14%, respectively. The average lateral size of the resulting graphene was 5μm. The Raman spectrum reveals that the graphene was defect free. The prepared graphene was utilized to fabricate flexible, transparent and conductive films on a PET substrate through a vacuum filtration and dry transfer method. The transparency of the film reached 83%, the resistance was 0.83kΩ/sq, and there was minimal variation, even after the film was bent more than 500 cycles.

Flexible and Stretchable Supercapacitors from Carbon Materials

Flexible and stretchable electronics have attracted increasing attention over the past decade. Compared with traditional electronics, which based on rigid silicon chips, flexible and stretchable electronics can conform to complex surfaces and sustain large deformations without sacrificing their performance. An important component of flexible or stretchable electronics is energy storage, such as supercapacitors or batteries. Existing stretchable supercapacitors are characterized by complicated fabrication processes, low stretchability and high cost, constraining their potential applications. Due to their electrical characteristics and large surface areas, carbon materials, such as activated carbon, carbon nanotubes, and graphene, have been investigated for supercapacitor applications. In this work, we report two new kinds of flexible and stretchable supercapacitors made from carbon nanotube and hybrid carbon materials using a facile and low-cost method. Stretchable electrodes were developed using vertically aligned carbon nanotube (CNT) forests grown on a silicon substrate by PECVD. The CNT forests were transferred to a prestretched elastomer substrate using a dry-transfer method. The CNT electrodes can be biaxially stretched up to 300% while maintaining good electrochemical performance. The area specific capacitance is enhanced almost 10x to more than 20mF/cm2. CNT forests can form novel micro- and nano- patterns under uniaxial or biaxial prestrains, making it possible to use them as conductive hierarchical scaffolds for functionalization with pseudocapacitive materials, such as MnO2 for better performance. Aerosol jet printing and self-organized origami were used to fabricate a stretchable RGO/CNT/PEDOT: PSS supercapacitor. The unique direct-printing method gives the supercapacitor outstanding mechanical properties. No delamination was observed at the interface between the printed film and the substrate during the large deformation process. The composite electrodes exhibited highly porous structures and high conductivity, which leads to good electrochemical performance: a large specific capacitance (150F/g), high rate capability (>95% at 5A/g compared to 0.1A/g), and high cyclability. Further, there was little performance degradation after hundreds of stretch-release cycles, indicating the excellent reliability of the stretchable devices. Our designs and methods open a new way for fabricating future energy storage devices with high deformability, high performance and low cost.

In situ observation of electrical property of thin-layer black phosphorus based on dry transfer method

The electrical property of thin-layer black phosphorus (BP) was explored using a simple dry transfer method, which greatly reduced the fabrication time to carry out electrical measurement starting from an initial state with little degradation. As a result, the as-prepared BP field-effect transistor (FET) exhibited a high on/off ratio exceeding 104 and a high hole mobility of 380 cm2/(V·s). The time-dependent electrical property of BP indicated a declining and recovering process, caused by the degradation and doping effect. Finally, it was demonstrated that the degradation and large hysteresis of BP FET could be modified by covering a thin Al2O3 layer.

Progress and Challenges in Transfer of Large-Area Graphene Films.

Graphene, the thinnest, strongest, and stiffest material with exceptional thermal conductivity and electron mobility, has increasingly received world‐wide attention in the past few years. These unique properties may lead to novel or improved technologies to address the pressing global challenges in many applications including transparent conducting electrodes, field effect transistors, flexible touch screen, single‐molecule gas detection, desalination, DNA sequencing, osmotic energy production, etc. To realize these applications, it is necessary to transfer graphene films from growth substrate to target substrate with large‐area, clean, and low defect surface, which are crucial to the performances of large‐area graphene devices. This critical review assesses the recent development in transferring large‐area graphene grown on Fe, Ru, Co, Ir, Ni, Pt, Au, Cu, and some nonmetal substrates by using various synthesized methods. Among them, the transfers of the most attention kinds of graphene synthesized on Cu and SiC substrates are discussed emphatically. The advances and the main challenges of each wet and dry transfer method for obtaining the transferred graphene film with large‐area, clean, and low defect surface are also reviewed. Finally, the article concludes the most promising methods and the further prospects of graphene transfer.

Construction of van der Waals magnetic tunnel junction using ferromagnetic layered dichalcogenide

We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe0.25TaS2. The vdW interlayer coupling at the Fe-intercalated plane of Fe0.25TaS2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material and the vdW junction.

(Invited) Vertical Field Effect Transistor Based on Graphene/Transition Metal Dichalcogenide Van Der Waals Heterostructure

Owing to the absence of a band gap, the direct applicability of graphene for transistors and logic devices is still limited. Recent studies have shown that the van der Waals heterostructure of graphene and other two-dimensional (2D) materials can be a great candidate to overcome these issues. The layered structure of these materials exhibits no direct chemical bonding between the different layers. Thus the vdW junction between these materials is, in principle, free from lattice mismatch issues and interface reactions. With these advantages, these vdW heterostructures has been demonstrated vertical field effect transistor (FET) operations as well as large current ON-OFF ratio [1]. However, realization of a large ON-OFF ratio simultaneously with a large ON current density has been still challenging. Here we fabricated exfoliated-graphene/MoS2/Ti vertical heterostructure including vdW interface built from freshly cleaved surface of graphene and MoS2 by using mechanical exfoliation and dry transfer method (Fig. 1(a)). The vertical transport of the device measured under the modulation of back gate voltage V G exhibits large modulation of its conductance (Fig. 1(b)). We obtained large current ON-OFF ratio of 105 as well as large ON current density of 104 A/cm2 [2]. Such large current modulation is attributed to the modulation of the Schottky barrier at graphene/MoS2 interface as schematically depicted in the right panel of Fig. 1(b). At the V G = +50 V, the Fermi level of the graphene is highest; therefore the Schottky barrier height at graphene/MoS2 junction is lowest. The reduction of Schottky barrier height makes device to be low resistance ON state. On the contrary, at the V G= -50 V, the Fermi level of the graphene is lowest; therefore the Schottky barrier height is highest. Here current flow in the device is greatly suppressed and works as OFF state. The importance of Schottky-barrier modulation on the performance of the transistor could be confirmed by the plot between the current ON-OFF ratio ION/IOFF vs. modulation of barrier height Δφ=φ max-φ min; these data are obtained from the series of devices with different thickness of MoS2 (Fig. 1(c)); results reveal correlation between two values [3]. These results suggest that even larger current modulation can be possible if one could have larger modulation of Schottky barrier height. For this purpose, we compared MoS2, MoSe2, and α-MoTe2 as transition metal dichalcogenide (TMD) materials in graphene/TMD/Ti heterostructure; these materials have different electron affinity and thus provide systematic change of band alignment to graphene (Fig. 1(d)). We show that the transport in the graphene/TMD junction is sensitive to the band alignment [4]. The V G dependence of the current revealed systematic change from MoS2 to MoTe2 (Fig. 1(e)). We demonstrated large current ON-OFF ratio and ON current density on MoSe2-based heterostructure; the value is comparable to that of MoS2-based device. These comparisons suggest that further improvement may be possible by optimizing device structure. We believe that our fabricated exfoliated-graphene/TMD/metal vertical heterostructure reveal superior performance to other existing graphene-based vertical transistors and present an important advance toward electronics applications.

Inoculation Preparation Affects Survival of Salmonella enterica on Whole Black Peppercorns and Cumin Seeds Stored at Low Water Activity

Salmonellosis has been increasingly associated with contaminated spices. Identifying inoculation and stabilization methods for Salmonella on whole spices is important for development of validated inactivation processes. The objective of this study was to examine the effects of inoculation preparation on the recoverability of Salmonella enterica from dried whole peppercorns and cumin seeds. Whole black peppercorns and cumin seeds were inoculated with S. enterica using one dry transfer method and various wet inoculation methods: immersion of spice seeds in tryptic soy broth (TSB) plus Salmonella for 24 h (likely leading to inclusion of Salmonella in native microbiota biofilms formed around the seeds), application of cells grown in TSB, and/or application of cells scraped from tryptic soy agar (TSA). Postinoculation seeds were dried to a water activity of 0.3 within 24 h and held for 28 days. Seeds were sampled after drying (time 0) and periodically during the 28 days of storage. Salmonella cells were enumerated by serial dilution and plated onto xylose lysine Tergitol (XLT4) agar and TSA. Recovery of Salmonella was high after 28 days of storage but was dependent on inoculation method, with 4.05 to 6.22 and 3.75 to 8.38 log CFU/g recovered from peppercorns and cumin seeds, respectively, on XLT4 agar. The changes in surviving Salmonella (log CFU per gram) from initial inoculation levels after 28 days were significantly smaller for the biofilm inclusion method ( + 0.142pepper, +0.186cumin) than for the other inoculation methods (–0.425pepper, –2.029cumin for cells grown on TSA; –0.641pepper, – 0.718cumin for dry transfer; – 1.998pepper for cells grown in TSB). In most cases, trends for reductions of total aerobic bacteria were similar to those of Salmonella. The inoculation method influenced the recoverability of Salmonella from whole peppercorns and cumin seeds after drying. The most stable inoculum strategies were dry transfer, 24-h incubation of Salmonella and spices in TSB (i.e., potential inclusion of Salmonella within native microbiota biofilms), and inoculation of Salmonella cells grown on TSA subsequent to drying. However, with the dry transfer method it was difficult to obtain the large amount of inoculum needed for inactivation studies.

Dry transfer of chemical-vapor-deposition-grown graphene onto liquid-sensitive surfaces for tunnel junction applications

We report a dry transfer method that can tranfer chemical vapor deposition (CVD) grown graphene onto liquid-sensitive surfaces. The graphene grown on copper (Cu) foil substrate was first transferred onto a freestanding 4 μm thick sputtered Cu film using the conventional wet transfer process, followed by a dry transfer process onto the target surface using a polydimethylsiloxane stamp. The dry-transferred graphene has similar properties to traditional wet-transferred graphene, characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and electrical transport measurements. It has a sheet resistance of 1.6 ∼ 3.4 kΩ/□, hole density of (4.1 ∼ 5.3) × 1012 cm−2, and hole mobility of 460 ∼ 760 cm2 V−1 s−1 without doping at room temperature. The results suggest that large-scale CVD-grown graphene can be transferred with good quality and without contaminating the target surface by any liquid. Mg/MgO/graphene tunnel junctions were fabricated using this transfer method. The junctions show good tunneling characteristics, which demonstrates the transfer technique can also be used to fabricate graphene devices on liquid-sensitive surfaces.

Influence of the transfer and chemical treatment of monolayer graphene grown for flexible transparent electrodes

It has become critically important to develop reliable method to transfer chemical vapor deposited (CVD) graphene from its growth substrate to the target substrate without leaving undesired polymer residues on the graphene surface. Here, we have found that for the two different transfer method – wet transfer using poly(methyl methacrylate) (PMMA) and dry transfer using polydimethylsiloxane (PDMS) as support layer, the amount of polymer residues and other impurities left on the graphene surface varies depending on the solvent used to remove those polymers. The exposure of the graphenes to different organic solvents such as acetone and chloroform resulted in different amount of polymer residues and impurities present on the graphene surface, which impact the electronic structure of the transferred graphene. It was found that the graphene obtained using the dry transfer method and acetone as solvent with a 2D to G (I2D/IG) intensity ratio of 4.58 and a 2D peak full width-half maximum (FWHM) of 24.66, which was higher than that using the wet transfer method and chloroform as solvent. These results showed that graphene was less affected by the polymer residues and impurities or the dry transfer method rather than the wet transfer method. In addition, using acetone rather than chloroform as solvent in the dry transfer method led to less contaminated graphene.

Highly flexible, all solid-state micro-supercapacitors from vertically aligned carbon nanotubes

We report a highly flexible planar micro-supercapacitor with interdigitated finger electrodes of vertically aligned carbon nanotubes (VACNTs). The planar electrode structures are patterned on a thin polycarbonate substrate with a facile, maskless laser-assisted dry transfer method. Sputtered Ni is used to reduce the in-plane resistance of the VACNT electrodes. An ionogel, an ionic liquid in a semi-solid matrix, is used as an electrolyte to form a fully solid-state device. We measure a specific capacitance of 430 μF cm−2 for a scan rate of 0.1 V s−1 and achieve rectangular cyclic voltammograms at high scan rates of up to 100 V s−1. Minimal change in capacitance is observed under bending. Mechanical fatigue tests with more than 1000 cycles confirm the high flexibility and durability of the novel material combination chosen for this device. Our results indicate that this scalable and facile fabrication technique shows promise for application in integrated energy storage for all solid-state flexible microdevices.

Transferring Few-Layer Graphene Sheets on Hexagonal Boron Nitride Substrates for Fabrication of Graphene Devices

We have developed a dry transfer method that allows graphene to be transferred from polymer- thyl-methacrylate (PMMA)/Si (silicon) substrates on commercially available hexagonal boron ni- tride (hBN) crystals. With this method we are able to fabricate graphene devices with little wrin- kles and bubbles in graphene sheets, but that do not degrade the electronic quality more than the SiO2 substrate does. For hBN to perform the function described above substrate cleanliness is critical to get high quality graphene devices. Using hBN as a substrate, graphene exhibits enhanced mobility, reduced carrier inhomogeneity, and reduced intrinsic doping compared to graphene on SiO2 substrate.

Carbon‐nanotube‐based flexible devices using a mechanical transfer method

AbstractWe describe a new method for the manufacture of single‐walled carbon nanotube (SWCNT)‐based electrical devices on flexible substrates. The method involves the deposition of a SWCNT films onto polydimethylsiloxane (PDMS) substrates from rigid substrates using the adhesive property of PDMS. This method is similar to the conventional dry transfer method, which is based on surface‐energy modification, except that our method involves mechanical transfer using the adhesive property of PDMS under peculiar conditions. The gas‐sensing characteristics of the resulting transferred SWCNT films are presented, showing, for example, that typical flexible sensors exhibit a sensitivity of 17.4% for 4‐ppm NO2 in a vacuum at room temperature. The performances of the devices are slightly reduced when they are bent to a curved profile with a bending radius of 2 cm (15.9%). The field‐emission properties are also investigated. From these emitters, the SWCNTs can be turned on with a field as low as 0.9 V/µm, and an emission current density of 0.75 mA/cm2 at 1.2 V/µm can be attained. Thus, a method for the fabrication of flexible devices is established, which should find practical applications in electronic devices.

Chemical Vapor Deposition-Derived Graphene with Electrical Performance of Exfoliated Graphene

While chemical vapor deposition (CVD) promises a scalable method to produce large-area graphene, CVD-grown graphene has heretofore exhibited inferior electronic properties in comparison with exfoliated samples. Here we test the electrical transport properties of CVD-grown graphene in which two important sources of disorder, namely grain boundaries and processing-induced contamination, are substantially reduced. We grow CVD graphene with grain sizes up to 250 μm to abate grain boundaries, and we transfer graphene utilizing a novel, dry-transfer method to minimize chemical contamination. We fabricate devices on both silicon dioxide and hexagonal boron nitride (h-BN) dielectrics to probe the effects of substrate-induced disorder. On both substrate types, the large-grain CVD graphene samples are comparable in quality to the best reported exfoliated samples, as determined by low-temperature electrical transport and magnetotransport measurements. Small-grain samples exhibit much greater variation in quality and inferior performance by multiple measures, even in samples exhibiting high field-effect mobility. These results confirm the possibility of achieving high-performance graphene devices based on a scalable synthesis process.

Fast and controllable fabrication of suspended graphene nanopore devices

A poly(methyl methacrylate) assisted dry transfer method was developed to transfer graphene microflake onto a suspended SiN chip in an effective and efficient way for further graphene nanopore drilling for DNA analysis. Graphene microflakes can be patterned by e-beam lithography to a designed shape and size on a large scale of a few thousands simultaneously. Subsequently, individual graphene microflakes can be picked up and transferred to a target hole on a suspended SiN membrane with 1 µm precision via a site-specific transfer-printing method. Nanopores with different diameters from 3 to 20 nm were drilled on the as-transferred graphene membrane in a transmission electron microscope. This method offers a fast and controllable way to fabricate graphene nanopores for DNA analyses.

Radio frequency nanowire resonators and in situ frequency tuning

We report the fabrication and characterization of radio frequency nanoelectromechanical resonators based on semiconductor nanowires with a dual-gate configuration. Suspended nanowire resonators were fabricated with little residual tension using a dry transfer method and lithographically defined metal electrodes. Fundamental flexural modes of the nanowire resonators were actuated and detected electrically at room temperature using the on-chip gates and a mixing circuit. The dual-gate setup in turn enabled us to tune the nanowire resonant frequency up or down in situ through elastic hardening and capacitive softening effects, respectively. Selective actuation of the out-of-plane or the in-plane vibrational mode was also reported.

Plastic transfer

A dry-transfer method can be used to print highly ordered arrays of nanowires onto flexible plastic substrates

Multicomponent Micropatterns or Carbon Nanotubes

The excellent optoelectronic, mechanical, and thermal properties of carbon nanotubes have made them very attractive for a wide range of potential applications. However, many applications require the growth of aligned/micropatterned carbon nanotubes. Based on our previous work on the aligned and micropatterned growth of carbon nanotubes, we have recently developed a novel approach towards the micropattern construction of perpendicularly-aligned carbon nanotubes by simply pressing a Scotch tape pre-patterned with a non-adhesive layer onto a non-patterned aligned carbon nanotube film, followed by peeling off the Scotch tape from the quartz substrate in a dry state. In conjunction with the region-specific surface modification, this dry contact transfer method has enabled us to produce various multicomponent carbon nanotube micropatterns in which different components are interposed in an intimate fashion. Examples include micropatterns with self-assembled non-aligned carbon nanotubes interdispersed into the discrete areas in the patterned structure of aligned carbon nanotubes and flexible polymer films with embedded aligned carbon nanotube networks. More recently, we have also developed a simple but effective template-free electroplating method for region-selective deposition of cobalt and nickel nanoparticles for patterned growth of carbon nanotubes. These carbon nanotube micropatterns in which multicomponents are interposed in a controllable fashion should be of significance to many nanotube based multifunctional systems.