Biswajit Saha

Bio: Biswajit Saha is an academic researcher from National Institute of Technology, Rourkela. The author has contributed to research in topics: Coating & Surface roughness. The author has an hindex of 8, co-authored 23 publications receiving 260 citations. Previous affiliations of Biswajit Saha include Seoul National University & Nanyang Technological University.

Highly Sensitive Bendable and Foldable Paper Sensors Based on Reduced Graphene Oxide.

Abstract: Over the past decade, the demand for high-performance wearable sensors has increased because of their capability for interaction with humans. Such sensors have typically been prepared on conventional substrates, such as silicon, PDMS, and copper mesh. In this work, we propose a class of wearable sensors fabricated from reduced graphene oxide (rGO) patterned paper substrates (rGO-paper). These rGO-paper sensors are highly sensitive to various deformations and capable of measuring bending and folding angles as small as 0.2° and 0.1°, respectively. We have demonstrated the applicability of these high-performance rGO-paper sensors by patterning rGO on kirigamis that can detect pulse and the motion of knees, wrists, and fingers. Finally, paper rings lined with rGO sensors were used to control a robotic hand, and an rGO-paper keyboard was used to light LEDs.

A review on the importance of surface coating of micro/nano-mold in micro/nano-molding processes

Abstract: Micro/nano hot-embossing and injection molding are two promising manufacturing processes for the mass production of workpieces bearing micro/nanoscale features. However, both the workpiece and micro/nano-mold are susceptive to structural damage due to high thermal stress, adhesion and friction, which occur at the interface between the workpiece and the mold during these processes. Hence, major constraints of micro/nano-molds are mainly attributed to improper replication and their inability to withstand a prolonged sliding surface contact because of high sidewall friction and/or high adhesion. Consequently, there is a need for proper surface coating as it can improve the surface properties of micro/nano-molds such as having a low friction coefficient, low adhesion and low wear rate. This review deals with the physical, mechanical and tribological properties of various surface coatings and their impact on the replication efficiency and lifetime of micro/nano-molds that are used in micro/nano hot-embossing and injection molding processes.

Anti-sticking behavior of DLC-coated silicon micro-molds

Abstract: Pure carbon- (C), nitrogen- (N) and titanium- (Ti) doped diamond-like carbon (DLC) coatings were deposited on silicon (Si) micro-molds by dc magnetron sputtering deposition to improve the tribological performance of the micro-molds. The coated and uncoated Si molds were used in injection molding for the fabrication of secondary metal-molds, which were used for the replication of micro-fluidic devices. The bonding structure, surface roughness, surface energy, critical load and friction coefficient of the DLC coatings were characterized with micro-Raman spectroscopy, atomic force microscopy (AFM), contact angle, microscratch and ball-on-disc sliding wear tests, respectively. It was observed that the doping conditions had significant effects on Raman peak positions, mechanical and tribological properties of the coatings. The G peak shifted toward a lower position with N and Ti doping. The DLC coating deposited with 1 sccm N2 flow rate showed the lowest G peak position and the smoothest surface. The surface energies of the pure carbon and Ti-doped DLC coatings were lower than that of the N-doped DLC, which was more significant at a higher N2 flow rate. In terms of adhesion and friction coefficient, it was observed that the Ti-doped DLC coating had the best performance. Ti incorporated in the DLC coating decreased the residual stress of the coating, which improved the adhesive strength of the coating with the Si substrate.

Replication performance of Si-N-DLC-coated Si micro-molds in micro-hot-embossing

Abstract: Micro-hot-embossing is an emerging technology with great potential to form micro- and nano-scale patterns into polymers with high throughput and low cost. Despite its rapid progress, there are still challenges when this technology is employed, as demolding stress is usually very high due to large friction and adhesive forces induced during the process. Surface forces are dominating parameters in micro- and nano-fabrication technologies because of a high surface-to-volume ratio of products. This work attempted to improve the surface properties of Si micro-molds by means of silicon- and nitrogen-doped diamond-like carbon (Si-N-DLC) coatings deposited by dc magnetron cosputtering on the molds. The bonding structure, surface roughness, surface energy, adhesive strength and tribological behavior of the coated samples were characterized with micro Raman spectroscopy, atomic force microscopy (AFM), contact angle measurement, microscratch test and ball-on-disk sliding tribological test, respectively. It was observed that the doping condition had a great effect on the performance of the coatings. The Si-N-DLC coating deposited with 5 × 10 −6 m 3 min −1 N2 had lowest surface roughness and energy of about 1.2 nm and 38.2 × 10 −3 Nm −1 , respectively, while the coatings deposited with 20 × 10 −6 and 25 × 10 −6 m 3 min −1 N2 showed lowest friction coefficients. The uncoated and Si-N-DLC-coated Si micro-molds were tested in a micro-hot-embossing process for a comparative study of their replication performance and lifetime. The experimental results showed that the performance of the Si micro-molds was improved by the Si-N-DLC coatings, and well-defined micro-features with a height of about 100 μm were fabricated successfully into cyclic olefin copolymer (COC) sheets using the Si-N-DLC-coated micro-molds.

Improvement in lifetime and replication quality of Si micromold using N:DLC:Ni coatings for microfluidic devices

Abstract: In this paper the effect of surface properties of micromolds on replication process was investigated by using nitrogen (N) and nickel (Ni) doped diamond-like carbon (N:DLC:Ni) coated and uncoated silicon (Si) micromolds. Hot embossing is one of the most popular replication technologies for low cost and mass production. However higher friction and adhesion in hot-embossing process can shorten the lifetime of micromolds. In the micro-hot-embossing process used for this study, the N:DLC:Ni coatings on the Si micromolds successfully increased the lifetime of the micromolds by 3–18 times. The N:DLC:Ni coatings were deposited on the Si micromolds by magnetron co-sputtering at various Ni target powers. The surface and tribological properties of the molds such as bonding structure, surface roughness, surface energy, adhesive strength, friction coefficient and wear resistance were characterized by micro-Raman spectroscopy, atomic force microscopy (AFM), contact angle measurement, micro-scratch test and ball-on-disk sliding test, respectively.

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics

Abstract: Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

Reduced graphene oxide today

Abstract: Reduced graphene oxide has similar mechanical, optoelectronic or conductive properties to pristine graphene because it possesses a heterogeneous structure comprised of a graphene-like basal plane that is additionally decorated with structural defects and populated with areas containing oxidized chemical groups. The graphene-like properties make reduced graphene oxide a highly desirable material to be used in a plethora of sensorial, biological, environmental or catalytic applications as well as optoelectronic and storage devices. To further advance the development of the existent technologies and to design novel and better applications based on reduced graphene oxide, it is first necessary to understand which synthetic routes and processing strategies are suitable to significantly boost specific properties of this material alone or as a component in various composites. Therefore, in this work, we review the most important categories of recent applications based on reduced graphene oxide, with the emphasis on the relationship between the enhanced composite/device functionality and methods used to synthesize, to functionalize and/or to process and to structure reduced graphene oxide.

Paper-Based Sensors for Gas, Humidity, and Strain Detections: A Review.

Abstract: Paper, as a flexible, low-cost, lightweight, tailorable, environmental-friendly, degradable, and renewable material, is emerging in electronic devices Especially, many kinds of paper-based (PB) sensors have been reported for wearable applications in recent years Among them, the PB gas, humidity, and strain sensors are widely studied for monitoring gas, humidity, and strain from the human body and the environment However, gas, humidity, and strain often coexist and interact, and the paper itself is hydrophilic and flexible, resulting in that it is still challenging to develop high-performance PB sensors specialized for gas, humidity, and strain detections Therefore, it is necessary to summarize and discuss them systematically In this review, we focus on summarizing the state-of-art studies of the PB gas, humidity, and strain sensors Specifically, the fabrications (electrodes and sensing materials) and applications of PB gas, humidity, and strain sensors are summarized and discussed The current challenges and the potential trends of PB sensors for gas, humidity, and strain detections are also outlined This review not only can help readers to understand the development status of the PB gas, humidity, and strain sensors but also is helpful for readers to find out and solve the problems in this field through comparative reading

Frictional Characteristics of Atomically-Thin Sheets

Abstract: Thin Friction The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. A universal trend is observed for the friction properties of thin films on weakly adhering substrates. Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length–dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets’ increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Micro hot embossing of thermoplastic polymers: a review

Abstract: Micro hot embossing of thermoplastic polymers is a promising process to fabricate high precision and high quality features in micro/nano scale. This technology has experienced more than 40 years development and has been partially applied in industrial production. Three modes of micro hot embossing including plate-to-plate, roll-to-plate and roll-to-roll have been successively developed to meet the increasing demand for large-area patterned polymeric films. This review surveys recent progress of micro hot embossing in terms of polymeric material behavior, embossing process and corresponding apparatus. Besides, challenges and innovations in mold fabrication techniques are comprehensively summarized and industrial applications are systematically cataloged as well. Finally, technical challenges and future trends are presented for micro hot embossing of thermoplastic polymers.