Daniel R. Hines

Printed Graphene Circuits

we have fabricated transparent electronic devices based on graphene materials with thickness down to one single atomic layer by the transfer printing method. The resulting printed graphene devices retain high field effect mobility and have low contact resistance. The results show that the transfer printing method is capable of high-quality transfer of graphene materials from silicon dioxide substrates, and the method thus will have wide applications in manipulating and delivering graphene materials to desired substrate and device geometries. Since the method is purely additive, it exposes graphene (or other functional materials) to no chemical preparation or lithographic steps, providing greater experimental control over device environment for reproducibility and for studies of fundamental transport mechanisms. Finally, the transport properties of the graphene devices on the PET substrate demonstrate the non-universality of minimum conductivity and the incompleteness of the current transport theory.

Nanotransfer printing of organic and carbon nanotube thin-film transistors on plastic substrates

A printing process for high-resolution transfer of all components for organic electronic devices on plastic substrates has been developed and demonstrated for pentacene (Pn), poly (3-hexylthiophene) and carbon nanotube (CNT) thin-film transistors (TFTs). The nanotransfer printing process allows fabrication of an entire device without exposing any component to incompatible processes and with reduced need for special chemical preparation of transfer or device substrates. Devices on plastic substrates include a Pn TFT with a saturation, field-effect mobility of 0.09cm2(Vs)−1 and on/off ratio approximately 104 and a CNT TFT which exhibits ambipolar behavior and no hysteresis.

Transfer printing methods for the fabrication of flexible organic electronics

A transfer printing method for fabricating organic electronics onto flexible substrates has been developed. The method relies primarily on differential adhesion for the transfer of a printable layer from a transfer substrate to a device substrate. The works of adhesion and cohesion for successful printing are discussed and developed for a model organic thin-film transistor device consisting of a polyethylene terephthalate (PET) substrate, gold (Au) gate and source/drain electrodes, a polymethylmethacrylate (PMMA) [or poly(4-vinylphenol)] dielectric layer, and a pentacene (Pn) organic semiconductor layer. The device components are sequentially printed onto the PET device substrate with no mixed processing steps performed on the device substrate. Optimum printing conditions for the Pn layer were determined to be 600psi and 120°C for 3min. A set of devices with a PMMA dielectric layer was measured as a function of channel length and exhibited a contact resistance corrected mobility of 0.237cm2∕Vs. This is la...

High-quality uniform dry transfer of graphene to polymers.

In this paper we demonstrate high-quality, uniform dry transfer of graphene grown by chemical vapor deposition on copper foil to polystyrene. The dry transfer exploits an azide linker molecule to establish a covalent bond to graphene and to generate greater graphene-polymer adhesion compared to that of the graphene-metal foil. Thus, this transfer approach provides a novel alternative route for graphene transfer, which allows for the metal foils to be reused.

Effect of initial resist thickness on residual layer thickness of nanoimprinted structures

Quantification and control of the residual layer thickness is a critical challenge facing nanoimprint lithography. This thickness must be known to within a few nanometers, yet there are very few nondestructive measurement techniques capable of extracting such information. Here we describe a specular x-ray reflectivity technique that can be used to not only quantify the thickness of the residual layer with sub-nm resolution, but also to extract the pattern height, the line-to-space ratio, and relative linewidth variations as a function of the pattern height. This is illustrated through a series of imprints where the initial film thickness is varied. For films with sufficient resist material to fill the mold, complete pattern filling is observed and the residual layer thickness is directly proportional to the initial film thickness. When there is insufficient resist material in the film to completely fill the patterns in the mold, a finite residual layer thickness of approximately 50–100A is still observed.

Poly(3-hexylthiophene) thin-film transistors with variable polymer dielectrics for transfer-printed flexible electronics

The fabrication of high quality organic thin-film transistors onto flexible, plastic substrates has been extended to include the polymeric semiconductor material poly(3-hexlythiophene). The transfer printing method is used to easily assemble these devices onto either polyethylene terephthalate (PET) or polycarbonate (PC) substrates. A PC dielectric layer is used in conjunction with the PC substrate while both poly(methyl methacrylate) and polystyrene dielectric layers are used in conjunction with the PET substrate. In all cases the mobility of the transfer-printed devices, 0.019–0.041 cm2/V s, is significantly higher than that of the unprinted reference devices (SiO2 dielectric layer on a Si substrate), 0.007 cm2/V s. The width-normalized contact resistance is also lower for the transfer-printed devices, 0.18 MΩ cm, as compared to that for the reference devices, 0.56 MΩ cm. For the devices reported, the threshold voltage becomes more positive as the polar component of the surface energy of the polymer die...

Nanoimprint pattern transfer quality from specular x-ray reflectivity

Specular x-ray reflectivity is used for high precision measurements of the pattern height, residual layer thickness, and the line-to-space ratio for parallel line and space patterns fabricated with nanoimprint lithography. The line-to-space ratio is profiled vertically to reveal relative linewidth variations as a function of the feature height. These relative linewidth variations are quantified through an external measure of the average pitch to fully define the line shape profile or cross section. An excellent fidelity of the nanoimprint pattern transfer process is quantified by comparing the line shape profiles of the mold to the imprinted pattern.

Evidence for internal stresses induced by nanoimprint lithography

The thermal embossing form of nanoimprint lithography is used to pattern arrays of nanostructures into three different polymer films. The shape of the imprinted patterns is characterized with nanometer precision using both x-ray scattering and reflectivity techniques. The time dependent response of the pattern shape at temperatures near the glass transition temperature reveals large levels of residual stress induced by the imprinting process. During the imprint, large shear fields are generated as the viscous polymer flows into the mold. If these shear distortions do not have time to relax during the imprinting, internal stresses are frozen into the final pattern. At elevated temperatures in the freestanding structures (once the mold has been separated from the imprint), there is an accelerated reduction in pattern height in the reverse direction from which the material originally flowed into the mold. Factors that influence this residual stress include the relative molecular mass or viscosity of the resi...

The Effect of Transfer Printing on Pentacene Thin-Film Crystal Structure

The thermal deposition and transfer printing method had been used to produce pentacene thin films on SiO2∕Si and plastic substrates poly(methyl methacrylate) (PMMA) and poly(vinyl pyridine), respectively. X-ray diffraction patterns of pentacene thin films showed reflections associated with highly ordered polycrystalline films and a coexistence of two polymorph phases classified by their d spacing, d(001): 14.4 and 15.4A. The dependence of the c-axis correlation length and the phase fraction on the film thickness and printing temperature were measured. A transition from the 15.4A phase towards 14.4A phase was also observed with increasing film thickness. An increase in the c-axis correlation length of approximately 12%–16% was observed for pentacene (Pn) films transfer printed onto a PMMA coated poly(ethylene terephthalate) substrate at 100–120°C as compared to as-grown Pn films on SiO2∕Si substrates. The transfer printing method is shown to be attractive for the fabrication of pentacene thin-film transist...

Transfer printing as a method for fabricating hybrid devices on flexible substrates

Printing methods are becoming important in the fabrication of flexible electronics. A transfer printing method has been developed for the fabrication of organic thin-film transistors (OTFT), capacitors, resistors and inductors onto plastic substrates. The method relies primarily on differential adhesion for the transfer of a printable layer from a transfer substrate to a device substrate. A range of materials applications is illustrated, including metals, organic semiconductors, organic dielectrics, nanotube and nanowire mats, a patterned inorganic semiconductor and graphene. Transfer printing can be used to create complex structures including many disparate materials sequentially printed onto the flexible substrate, with no mixed processing steps performed on the device substrate. Specifically, the fabrication and performance of model OTFT devices consisting of a polyethylene terephthalate (PET) substrate, gold (Au) gate and source/drain electrodes, a poly(methyl methacrylate) (PMMA) dielectric layer and either a pentacene (Pn) or a poly(3- hexylthiophene) (P3HT) organic semiconductor layer will be presented. These transfer printed OTFTs on plastic outperform non-printed devices on a Si substrate with a SiO2 dielectric layer (SiO2/Si). Transfer printed Pn OTFTs on a plastic substrate have exhibited mobilities of 0.237 cm2/Vs, compared to non-printed Pn OTFTs on a SiO2/Si substrate with mobilities of 0.1 cm2/Vs. Transfer printed P3HT TFTs on a plastic substrate have exhibited mobilites of 0.04 cm2/Vs, compared to non-printed P3HT TFTs on a SiO2/Si substrate with mobilities of 0.007 cm2/Vs.