PMMA Residues Research Articles

A-360 Reproducible Graphene Sensor with Desiccated Biology

Abstract Background Graphene, a two-dimensional material composed of carbon atoms, has garnered significant attention for its potential application in biosensors due to its unique electrical properties. Despite its promising attributes, however, the non-reproducible nature of graphene biosensors has hindered their widespread commercialization. This issue arises from the difficulty in obtaining a clean graphene layer due to: photoresist such as PMMA residues that effect the performance of graphene; etching process which affect the physical and chemical properties of graphene; variation in the coating of bio-receptors on to graphene. All these issues affect reliable and accurate sensor performance. These challenges have made it difficult for graphene biosensors to reach their full potential as commercial products and have limited their widespread adoption. Hememics has developed a unique process for synthesizing and fabricating graphene-based biosensors. This represents a major breakthrough in the commercialization of graphene-based biosensors and will likely lead to widespread adoption of these devices in the medical and biotechnology industries. Methods HemChip™ contains multiple sensor areas that are coated with ultra-sensitive graphene. Using Hememics’ unique proprietary process, the graphene layer is functionalized with bioreceptors such as aptamers and stabilized with HemSol™ to allow room temperature storage. The relative difficulty of stabilizing aptamers on a hydrophobic graphene substrate has been overcome by using HemSol™ to secure its structure during dry storage. Each sensor on the biochip allows independent functionalization with a bioreceptor. These sensors coated with bioreceptors are freeze-dried and stored in a sealed pouch at room temperature until use. Results We have tested over 2000 samples using our modified process. The functionality of the graphene-based biosensor is evaluated by measuring the distribution of Dirac points. The results show that the distribution follows a Gaussian distribution with a median of 80 mV and a very narrow spread of approximately 35 mV. This indicates that the majority of the Dirac points are tightly clustered around the median value of 80 mV. Furthermore, one standard deviation of the Dirac points fall within the range of 50 mV to 120 mV. In contrast, the standard published process of producing graphene-based biosensor shows a much wider distribution of Dirac points, ranging from 250–1500 mV. Conclusion Hememics has developed a production process for making graphene-based biosensors with desiccated detection biology that deliver repeatable performance data. The results of the Dirac point distribution demonstrate the high precision and accuracy for the functionalized biosensors. This combined with the ability to mass produce the biosensor, make graphene-based biosensors an attractive molecular and immuno-based detection tool. Additionally, the results suggest that the Hememics process can be adapted to produce functional graphene-based biosensors for wide ranging bio detection applications.

Optimizing PMMA solutions to suppress contamination in the transfer of CVD graphene for batch production.

Mass production and commercial adoption of graphene-based devices are held back by a few crucial technical challenges related to quality control. In the case of graphene produced by chemical vapor deposition, the transfer process represents a delicate step that can compromise device performance and reliability, thus hindering industrial production. In this context, the impact of poly(methyl methacrylate) (PMMA), the most common support material for transferring graphene from the Cu substrate to any target surface, can be decisive in obtaining reproducible sample batches. Although effective in mechanically supporting graphene during the transfer, PMMA solutions needs to be efficiently designed, deposited, and post-treated to serve their purpose while minimizing potential contaminations. Here, we prepared and tested PMMA solutions with different average molecular weight (AMW) and weight concentration in anisole, to be deposited by spin coating. Optical microscopy and Raman spectroscopy showed that the amount of PMMA residues on transferred graphene is proportional to the AMW and concentration in the solvent. At the same time, the mechanical strength of the PMMA layer is proportional to the AMW. These tests served to design an optimized PMMA solution made of a mixture of 550,000 (550k) and 15,000 (15k) AMW PMMA in anisole at 3% concentration. In this design, PMMA-550k provided suitable mechanical strength against breakage during the transfer cycles, while PMMA-15k promoted depolymerization, which allowed for a complete removal of PMMA residues without the need for any post-treatment. An XPS analysis confirmed the cleanness of the optimized process. We validated the impact of the optimized PMMA solution on the mass fabrication of arrays of electrolyte-gated graphene field-effect transistors operating as biosensors. On average, the transistor channel resistance decreased from 1860 to 690 Ω when using the optimized PMMA. Even more importantly, the vast majority of these resistance values are distributed within a narrow range (only ca. 300 Ω wide), in evident contrast with the scattered values obtained in non-optimized devices (about 30% of which showed values above 1 MΩ). These results prove that the optimized PMMA solution unlock the production of reproducible electronic devices at the batch scale, which is the key to industrial production.

Water mediated electrochemical conversion of PMMA and other organic residues into graphene and carbon materials

This paper proposes a new thermal, electrical, and water assisted reaction (TEAWAR) technique to convert PMMA and organic residues into graphene and carbonaceous materials. Graphene and other carbon based materials have attracted considerable research interest due to their unique and enhanced properties. Graphene is a particularly promising material for electronic and sensing applications, although preparation is cumbersome requiring high vacuum and other sophisticated facilities to produce quality graphene layers. Graphene and other carbon based materials are commonly prepared from organic and polymer precursors under high thermal conditions. This paper proposes a facile method to prepare graphene and other carbonaceous materials from poly(methyl methacrylate) (PMMA) polymer. PMMA is converted into graphene using a PMMA coated conducting substrate as one electrode in a two electrode electrochemical cell with water electrolyte, where conversion success depends on applied potential, time, temperature, and PMMA layer thickness. We converted PMMA coated on conducting substrates into graphene using 200 V potential for more than 1 h at 50 °C. This technique's suitability for organic residues conversion into carbon was also tested and found to be feasible to convert onion peel and other waste materials into carbon. Converted graphene was applied for photosensing and hydrogen evolution reaction (HER) applications. The formed graphene exhibited good catalytic property comparable to various graphene related materials.

A Novel Methodology of Using Nonsolvent in Achieving Ultraclean Transferred Monolayer MoS2

AbstractThe transfer of monolayer molybdenum disulfide (1L‐MoS2) onto any target substrates is inevitable for the next generation of optoelectronic devices such as flexible electronics. However, the existing post‐transfer treatments are ineffective for the complete removal of poly(methyl methacrylate) (PMMA) polymer, which is usually used as carrier polymer in the wet‐transfer method. The presence of PMMA residues seriously degrades the intrinsic properties of any 2D materials. Several new cleaning methods such as annealing, ozone cleaning, and acetone treatment adopted in this report are found to be ineffective for the complete removal of PMMA residues on the transferred 1L‐MoS2 film. A new chemical route is developed and demonstrated with a PMMA nonsolvent, ethanol, and ultraclean transferred 1L‐MoS2 is obtained after 96 h hot ethanol treatment. An interfacial diffusion model is proposed for the mechanism of PMMA removal from the 1L‐MoS2 surface. To observe the effect of cleaning process on electrical properties, 1L‐MoS2 field‐effect transistor (FET) devices are fabricated. An enhancement of 80%–85% in the electron mobility is achieved for ultraclean 1L‐MoS2. One order improvement in the FET parameters such as ON/OFF ratio and the subthreshold slope is also observed. It is believed that this novel method can also be applicable for other 2D materials.

Defects Produced during Wet Transfer Affect the Electrical Properties of Graphene.

Graphene has been widely used due to its excellent electrical, mechanical and chemical properties. Defects produced during its transfer process will seriously affect the performance of graphene devices. In this paper, single-layer graphene was transferred onto glass and silicon dioxide (SiO2) substrates by wet transfer technology, and the square resistances thereof were tested. Due to the different binding forces of the transferred graphene surfaces, there may have been pollutants present. PMMA residues, graphene laminations and other defects that occurred in the wet transfer process were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. These defects influenced the square resistance of the produced graphene films, and of these defects, PMMA residue was the most influential; square resistance increased with increasing PMMA residue.

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases. While preferential n-type doping was observed in graphene domains in nitrogen-containing plasmas, preferential p-type doping was observed at grain boundaries in boron-containing plasmas.

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases.

Impact of Post‐Lithography Polymer Residue on the Electrical Characteristics of MoS2 and WSe2 Field Effect Transistors

AbstractThe residue of common photo‐ and electron‐beam resists, such as poly(methyl methacrylate) (PMMA), is often present on the surface of 2D crystals after device fabrication. The residue degrades device properties by decreasing carrier mobility and creating unwanted doping. Here, MoS2 and WSe2 field effect transistors (FETs) with residue are cleaned by contact mode atomic force microscopy (AFM) and the impact of the residue on: 1) the intrinsic electrical properties, and 2) the effectiveness of electric double layer (EDL) gating are measured. After cleaning, AFM measurements confirm that the surface roughness decreases to its intrinsic state (i.e., ≈0.23 nm for exfoliated MoS2 and WSe2) and Raman spectroscopy shows that the characteristic peak intensities (E2g and A1g) increase. PMMA residue causes p‐type doping corresponding to a charge density of ≈7 × 1011 cm−2 on back‐gated MoS2 and WSe2 FETs. For FETs gated with polyethylene oxide (PEO)76:CsClO4, removing the residue increases the charge density by 4.5 × 1012 cm−2, and the maximum drain current by 247% (statistically significant, p < 0.05). Removing the residue likely allows the ions to be positioned closer to the channel surface, which is essential for achieving the best possible electrostatic gate control in ion‐gated devices.

Facile process to clean PMMA residue on graphene using KrF laser annealing

Persistent PMMA residue formed during a graphene transfer has been a culprit in the optimization of graphene device performance. We demonstrated a facile process to remove the PMMA residue using pulsed KrF laser annealing system at H2/Ar ambient. 10min laser annealing at 248nm could remove the PMMA residue as well as the methoxy and carboxyl function groups without causing noticeable damage to the graphene.

Optimized poly(methyl methacrylate)-mediated graphene-transfer process for fabrication of high-quality graphene layer

Graphene grown on a copper (Cu) substrate by chemical vapor deposition (CVD) is typically required to be transferred to another substrate for the fabrication of various electrical devices. PMMA-mediated wet process is the most widely used method for CVD-graphene-transfer. However, PMMA residue and wrinkles that inevitably remain on the graphene surface during the transfer process are critical issues degrading the electrical properties of graphene. In this paper, we report on a PMMA-mediated graphene-transfer method that can effectively reduce the density and size of the PMMA residue and the height of wrinkles on the transferred graphene layer. We found out that acetic acid is the most effective PMMA stripper among the typically used solutions to remove the PMMA residue. In addition, we observed that an optimized annealing process can reduce the height of the wrinkles on the transferred graphene layer without degrading the graphene quality. The effects of the suggested wet transfer process were also investigated by evaluating the electrical properties of field-effect transistors fabricated on the transferred graphene layer. The results of this work will contribute to the development of fabrication processes for high-quality graphene devices, given that the transfer of graphene from the Cu substrate is essential process to the application of CVD-graphene.

Influences of Polymer Residues on the Growth Properties of Pentacene Thin Film on Graphene Substrates

Understanding of the growth properties of organic thin film (OTF) on graphene surface is beneficial to develop high-performance organic electronic devices based on OTF and graphene electrodes. In this work, we investigated the growth behaviors of pentacene on the poly(methyl methacrylate) (PMMA)-assisted wet-transfer graphene substrates. Especially, the influence of PMMA residues and graphene wrinkles on the initial nucleation behavior and subsequent aggregation of pentacene were studied in detail. Pentacene adopts standing-up geometry on the as-transferred graphene substrates. The molecules prefer to initially reside beside graphene wrinkles where the density of PMMA residues is found to be larger than the flat regions of graphene and then the subsequent deposited pentacene was packed face-to-face and developed into one-dimensional fiberlike domains with a fiber length of few micrometers. Controlled experiments and detailed surface analysis of graphene reveal that the hydroxyl groups of polymer residues ...

Determination of PMMA Residues on a Chemical-Vapor-Deposited Monolayer of Graphene by Neutron Reflection and Atomic Force Microscopy

Chemical vapor deposition (CVD) is now a well-established method for creating monolayer graphene films. In this method, poly(methyl methacrylate) (PMMA) films are often coated onto monolayer graphene films to make them mechanically robust enough for transfer and further handling. However, it is found that PMMA is hard to remove entirely, and any residual polymers remaining can affect graphene's properties. We demonstrate here a method to determine the amount of PMMA remaining on the graphene sheet fabricated from CVD by a combined study of Raman scattering, atomic force microscopy, and neutron reflection. Neutron reflectivity is a powerful technique which is particularly sensitive to any interfacial structure, so it is able to investigate the density profile of the residual PMMA in the direction perpendicular to the graphene film surface. After the standard process of PMMA removal by acetone-IPA cleaning, we found that the remaining PMMA film could be represented as a two-layer model: an inner layer with a thickness of 17 Å and a roughness of 1 Å mixed with graphene and an outer diffuse layer with an average thickness of 31 Å and a roughness of 4 Å well mixed with water. On the basis of this model analysis, it was demonstrated that the remaining PMMA still occupied a significant fraction of the graphene film surface.

Improved efficiency of graphene/Si Schottky junction solar cell based on back contact structure and DUV treatment

A graphene/Si Schottky junction solar cell is commonly fabricated by using the top-window structure. However, reported devices have many drawbacks such as a small active area of 0.11 cm2, s-shape in the J-V curves, recombination process of charge carriers at the graphene/textured Si interface, high cost and a complex fabrication process. Here, we report a novel graphene/Si Schottky junction solar cell with a back contact-structure, which has benefits of a simpler fabrication process, lower fabrication cost, and larger active area in comparison with a device fabricated with the previous structure. Additionally, we found that the PMMA residue left on graphene surfaces is the key to eliminate the s-shape in the J-V curves. Thus, the deep UV treatment of the CVD graphene is applied within the wet transfer process to effectively remove the PMMA residue, suppress the behavior of s-shaped kink in J-V curves and enhance the solar cell efficiency. As a result, the recorded power conversion efficiency of 10% is achieved for graphene/textured Si devices without chemical doping and anti-reflection coating, and this value is improved to 14.1% after applying chemical doping. Doped devices also show great stability and retain 84% of the efficiency after 9 days storage in air.

Favorable Face‐on Orientation of a Conjugated Polymer on Roll‐to‐Roll‐Transferred Graphene Interface

AbstractThe use of dry transferred graphene as a templating layer to enhance face‐to‐face stacking in poly(3‐hexylthiophene) (P3HT) systems which is widely used for organic optoelectronics is investigated. In contrast to conventional poly(methyl methacrylate) (PMMA) assisted wet transferred graphene, dry transferred graphene is found in the current work to be quite suitable for use in the roll‐to‐roll process due to its lack of PMMA residue, whose removal would require high‐temperature annealing. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) is used to determine the percentages of P3HT molecules adopting a face‐on orientation on the various substrates tested. When a P3HT film with a thickness of 30 nm is produced, the face‐on populations of P3HT are prominent on both the dry and wet transferred graphene layers. As the film thickness is increased to 50 nm, the face‐on population decreases on the wet transferred graphene surface, but retains high levels on the dry transferred graphene. GIWAXS, near‐edge X‐ray absorption fine structure, and atomic force microscopy data are combined to propose schematic models for the molecular stacking of P3HTs on the two differently transferred graphene surfaces.

High performance MoS2 TFT using graphene contact first process

An ohmic contact of graphene/MoS2 heterostructure is determined by using ultraviolet photoelectron spectroscopy (UPS). Since graphene shows a great potential to replace metal contact, a direct comparison of Cr/Au contact and graphene contact on the MoS2 thin film transistor (TFT) is made. Different from metal contacts, the work function of graphene can be modulated. As a result, the subthreshold swing can be improved. And when Vg<VFB, the intrinsic graphene changes into p-type, so graphene contact can achieve lower off current by lowering the Fermi level. To further improve the performance of MoS2 TFT, a new method using graphene contact first and MoS2 layer last process that can avoid PMMA residue and high processing temperature is applied. MoS2 TFT using this method shows on/off current ratio up to 6×106 order of magnitude, high mobility of 116 cm2/V-sec, and subthreshold swing of only 0.515 V/dec.

Study of copper-phthalocyanine and pentacene film growth on transferred graphene: The influence of polymer residues

Wet transferring of chemical vapor deposition (CVD) graphene with the assistance of poly(methyl methacrylate) (PMMA) usually leaves polymer contaminations onto graphene surface. Knowledge about the influence of PMMA residues on organic film growth properties is crucial for graphene-related organic electronic devices. In this study, we investigate growth properties of coper-phthalocyanine (CuPc) and pentacene on PMMA contaminated graphene substrate. Grain size and morphologies of CuPc and pentacene films were found to be governed by the density of PMMA residues on graphene. Nevertheless, the molecular packing structures of CuPc and pentacene respond differently with the density with PMMA residues. CuPc molecules tend to nucleated into grains with molecular π-π stacking parallel to graphene surface, which is irrespective of the PMMA residues. Whereas the packing behavior of pentacene on PMMA-transferred CVD graphene is sensitive to the density of PMMA residues. Molecular π-π stacking perpendicular to graphene is observed when pentacene was deposited onto as-transferred graphene. Parallel mode was observed only on the substrates without PMMA residues.

Reprint of: Improvements on thermal stability of graphene and top gate graphene transistors by Ar annealing

Thermal annealing of graphene was studied to improve the thermal stability. During the annealing under an Ar atmosphere with temperature from 100 °C to 600 °C, graphene exhibits partial removal of PMMA residues, low density of defect cracks in SEM images and relatively low ID/IG ratios in Raman spectrum probing. For a top-gated graphene transistor with thermal annealing, it performs high carrier mobility up to 3500 cm2 V−1 s−1 and slightly asymmetric bipolar behaviours as well as slight Dirac point variation within ±0.2 V.

Characterization and electrolytic cleaning of poly(methyl methacrylate) residues on transferred chemical vapor deposited graphene

Poly(methyl methacrylate) (PMMA) residue has long been a critical challenge for practical applications of the transferred chemical vapor deposited (CVD) graphene. Thermal annealing is empirically used for the removal of the PMMA residue; however experiments imply that there are still small amounts of residues left after thermal annealing which are hard to remove with conventional methods. In this paper, the thermal degradation of the PMMA residue upon annealing was studied by Raman spectroscopy. The study reveals that post-annealing residues are generated by the elimination of methoxycarbonyl side chains in PMMA and are believed to be absorbed on graphene via the π–π interaction between the conjugated unsaturated carbon segments and graphene. The post-annealing residues are difficult to remove by further annealing in a non-oxidative atmosphere due to their thermal and chemical stability. An electrolytic cleaning method was shown to be effective in removing these post-annealing residues while preserving the underlying graphene lattice based on Raman spectroscopy and atomic force microscopy studies. Additionally, a solution-gated field effect transistor was used to study the transport properties of the transferred CVD graphene before thermal annealing, after thermal annealing, and after electrolytic cleaning, respectively. The results show that the carrier mobility was significantly improved, and that the p-doping was reduced by removing PMMA residues and post-annealing residues. These studies provide a more in-depth understanding on the thermal annealing process for the removal of the PMMA residues from transferred CVD graphene and a new approach to remove the post-annealing residues, resulting in a residue-free graphene.

Direct Observation of Poly(Methyl Methacrylate) Removal from a Graphene Surface

Poly(methyl methacrylate) (PMMA) is commonly used as a temporary support layer for chemical vapor deposition (CVD) graphene transfer; it is then removed by a chemical or thermal treatment. Regardless of the method used for PMMA removal, polymer residues are left on the graphene surface, which alter its intrinsic properties. A method based on isotope labeling of PMMA and time-of-flight secondary ion mass spectrometry (ToF-SIMS) has now been developed to identify, locate, and quantify these residues. It is shown that vacuum annealing does not completely remove the PMMA residues but, instead, transforms them into amorphous carbon. In contrast, air annealing under optimized conditions generates a PMMA-free surface with limited damage to the graphene structure. This cleaned graphene surface demonstrates low friction which is comparable with that of pristine graphene film.

Graphene on graphene formation from PMMA residues during annealing

PMMA is a common support material for transferring graphene between substrates. However, PMMA residues typically remain on the graphene sheet after the transfer process. A high temperature annealing process is commonly applied to reduce the amount of PMMA residues. It is also known that high temperature annealing of PMMA causes the PMMA to graphitize, which has been used as a method to synthesize graphene on metal substrates. In this letter we show the development of additional graphene layers during high temperature annealing, which occurs on a single, clean, graphene sheet. The additional graphene is nucleated from the decomposition products of PMMA residues.