Microwave bottom-up route for size-tunable and switchable photoluminescent graphene quantum dots using acetylacetone: New platform for enzyme-free detection of hydrogen peroxide

Umrao, Sima,Jang, Min-Ho,Oh, Jung-Hwan,Kim, Guntae,Sahoo, Sumanta,Cho, Yong-Hoon,Srivastva, Anchal,Oh, Il-Kwon Elsevier 2015 Carbon Vol.81 No.-

<P><B>Abstract</B></P> <P>We report a microwave sequential bottom-up route to produce green and blue luminescent graphene quantum dots (g-GQDs and b-GQDs) with size-tunable and switchable functionalities by tailoring the diameter size and functional groups via microwave carbonization and aromatization processes from acetylacetone as a starting organic solvent. The b-GQDs as the final product show only one emission peak at 433nm and pH-independent blue luminescence, because two-step microwave irradiation could reduce the size and the oxygen-functional groups of the g-GQDs as an intermediate product. Also, the b-GQDs provide an exemplar enzyme-free platform for hydrogen peroxide detection through the electrochemical sensing due to much higher electron density and electron donating properties. In contrast, the g-GQDs show two different switchable photoluminescent emissions at ∼460nm (P1) and ∼500nm (P2): the P1 emission with sky-blue fluorescence originates from randomly conjugated oxygen-functional groups on the basal plane and/or edge of the g-GQDs and the P2 emission with green fluorescence results from quasi-molecular fluorophores formed by the electronic coupling of carboxylic acid groups.</P>

Microwave-Assisted Synthesis of Boron and Nitrogen co-doped Reduced Graphene Oxide for the Protection of Electromagnetic Radiation in Ku-Band

Umrao, Sima,Gupta, Tejendra K.,Kumar, Shiv,Singh, Vijay K.,Sultania, Manish K.,Jung, Jung Hwan,Oh, Il-Kwon,Srivastava, Anchal American Chemical Society 2015 ACS APPLIED MATERIALS & INTERFACES Vol.7 No.35

<P>The electromagnetic interference (EMI) shielding of reduced graphene oxide (MRG), B-doped MRG (BMRG), N-doped MRG (N-MRG), and B-N co-doped MRG (B-N-MRG) have been studied in the Ku-band frequency range (12.8-18 GHz). We have developed a green, fast, and cost-effective microwave assisted route for synthesis of doped MRG. B-N-MRG shows high electrical conductivity in comparison to MRG, B-MRG and N-MRG, which results better electromagnetic interference (EMI) shielding ability. The co-doping of B and N significantly enhances the electrical conductivity of MRG from 21.4 to 124.4 Sm-1 because N introduces electrons and B provides holes in the system and may form a nanojunction inside the material. Their temperature-dependent electrical conductivity follows 2D-variable range hopping (2D-VRH) and Efros-Shldovskii-VRH (ES-VRH) conduction model in a low temperature range (T < 50 K). The spatial configuration of MRG after doping of B and N enhances the space charge polarization, natural resonance, dielectric polarization, and trapping of EM waves by internal reflection leading to a high EMI shielding of -42 dB (-99.99% attenuation) compared to undoped MR.G (-28 dB) at a critical thickness of 1.2 mm. Results suggest that the B-N-MRG has great potential as a candidate for a new type of EMI shielding material useful in aircraft, defense industries, communication systems, and stealth technology.</P>

A homogeneous atomic layer MoS2(1-x)Se2x alloy prepared by low-pressure chemical vapor deposition, and its properties

Umrao, S.,Jeon, J.,Jeon, S.,Choi, Y.,Lee, S. Royal Society of Chemistry 2017 Nanoscale Vol.9 No.2

<P>We report the growth of large-area monolayer MoS2(1-x)Se2x alloys with controlled morphologies using a low-pressure chemical vapor deposition (CVD) method. MoS2(1-x)Se2x alloys with different morphologies, created using the same growth time, have been observed by controlling the gaseous MoO3 precursor on substrates placed in regions with different temperatures. TEM observations clearly reveal that the as-synthesized monolayer MoS2(1-x)Se2x alloy is crystalline, with a hexagonal structure. XPS, Raman mapping, and EDS mapping clearly show the homogeneous substitution of similar to 2 atomic weight % Se through the whole crystal. Compared with a pristine CVD-grown monolayer of MoS2, the optical band gap differs by 4.52%, from 1.77 eV to 1.69 eV. Additionally, back-gated transistors fabricated on the monolayer MoS2(1-x)Se2x alloy exhibit n-type behavior at a current on/off ratio of similar to 10(4) and a high mobility value of 8.4 cm(2) V(-1)s(-1).</P>

Microwave-assisted boron and nitrogen co-doped reduced graphene oxide as a transparent conductive electrode

Umrao, Sima,Mishra, Himanshu,Srivastava, Anchal,Lee, Sungjoo American Institute of Physics 2017 Applied Physics Letters Vol.111 No.2

Device-to-Device Communication from Control and Frequency Perspective: A Composite Review

Umrao, Sachin,Roy, Abhishek,Saxena, Navrati Informa UK (TaylorFrancis) 2017 IETE TECHNICAL REVIEW Vol.34 No.3

<P>Device-to-device (D2D) communication facilitates two users in close proximity to communicate directly, bypassing the Evolved Node B (eNB). It provides several advantages like increased spectral efficiency, energy efficiency, decreased transmission delay, and traffic offloading from eNBs. Native support of D2D communications in the emerging fifth generation cellular systems raises significant challenges, like interference, mode selection, resource allocation, and security. Ratification of standards and collaborative research works between industries and academia are now focusing to gradually resolve these challenges. In this paper, we present a novel survey of D2D communications. Our survey provides a new classification of major research works in D2D communications on the basis of frequency and control. Subsequently, major D2D architectures, protocols, and relevant applications are discussed. We also make a detailed review of the analysis techniques and point out open research challenges.</P>

Multilevel Hierarchical Caching for Efficient Wireless Video Distribution

Umrao, Sachin,Roy, Abhishek,Saxena, Navrati Taylor Francis 2017 IETE journal of research Vol.63 No.2

Second-order statistics of natural frequencies of smart laminated composite plates with random material properties

Singh, B.N.,Umrao, Atul,Shukla, K.K.,Vyas, N. Techno-Press 2008 Smart Structures and Systems, An International Jou Vol.4 No.1

Nowadays developments in the field of laminated composite structures with piezoelectric have attracted significant attention of researchers due to their wide range of applications in engineering such as sensors, actuators, vibration suppression, shape control, noise attenuation and precision positioning. Due to large number of parameters associated with its manufacturing and fabrication, composite structures with piezoelectric display a considerable amount of uncertainty in their material properties. The present work investigates the effect of the uncertainty on the free vibration response of piezoelectric laminated composite plate. The lamina material properties have been modeled as independent random variables for accurate prediction of the system behavior. System equations have been derived using higher order shear deformation theory. A finite element method in conjunction with Monte Carlo simulation is employed to obtain the secondorder statistics of the natural frequencies. Typical results are presented for all edges simply supported piezoelectric laminated composite plates to show the influence of scattering in material properties on the second order statistics of the natural frequencies. The results have been compared with those available in literature.

Second-order statistics of natural frequencies of smart laminated composite plates with random material properties

B. N. Singh,Atul Umrao,K. K. Shukla,N. Vyas 국제구조공학회 2008 Smart Structures and Systems, An International Jou Vol.4 No.1

Nowadays developments in the field of laminated composite structures with piezoelectric have attracted significant attention of researchers due to their wide range of applications in engineering such as sensors, actuators, vibration suppression, shape control, noise attenuation and precision positioning. Due to large number of parameters associated with its manufacturing and fabrication, composite structures with piezoelectric display a considerable amount of uncertainty in their material properties. The present work investigates the effect of the uncertainty on the free vibration response of piezoelectric laminated composite plate. The lamina material properties have been modeled as independent random variables for accurate prediction of the system behavior. System equations have been derived using higher order shear deformation theory. A finite element method in conjunction with Monte Carlo simulation is employed to obtain the secondorder statistics of the natural frequencies. Typical results are presented for all edges simply supported piezoelectric laminated composite plates to show the influence of scattering in material properties on the second order statistics of the natural frequencies. The results have been compared with those available in iterature.

카본우븐패브릭 기반 2D 구조의 Ti₃C₂T_x 배터리음극소재

남상희 ( Sanghee Nam ),시마엄라오 ( Sima Umrao ),오세웅 ( Saewoong Oh ),오일권 ( Il-kwon Oh ) 한국복합재료학회 2019 Composites research Vol.32 No.5

2D 전이금속 탄화물(MXenes) 가운데, 타이타늄 기반의 Ti₃C₂는 뛰어난 전기전도성과 전기화학적 특성 및 표면작용기의 영향으로 이차 전지와 슈퍼캐패시터와 같은 에너지저장장치의 유망한 전극 물질로 각광받고 있다. 전극으로서 Ti₃C₂의 사용은 이온이 반응할 수 있는 표면적을 넓혀줄 뿐만 아니라, 이온의 확산 거리를 줄여주고, 전하의 운동을 향상시켜준다. 이 연구에서, 효율적으로 MAX phase로부터 Ti₃C₂를 합성하는 방법을 통해 리튬이온배터리에서 MXene기반의 전극 물질을 위한 새로운 방향을 제시하고자 한다. Two dimensional transition metal carbides and/or nitrides, known as MXenes, are a promising electrode material in energy storage due to their excellent electrical conductivity, outstanding electrochemical performance, and abundant functional groups on the surface. Use of Ti₃C₂ as electrode material has significantly enhanced electrochemical performance by providing more chemically active interfaces, short ion-diffusion lengths, and improved charge transport kinetics. Here, we reports the efficient method to synthesize Ti₃C₂ from MAX phase, and opens new avenues for developing MXene based electrode materials for Lithium-Ion batteries.

Self-aligned and hierarchically porous graphene-polyurethane foams for acoustic wave absorption

Oh, Jung-Hwan,Lee, Hyeong Rae,Umrao, Sima,Kang, Yeon June,Oh, Il-Kwon Elsevier 2019 Carbon Vol.147 No.-

<P><B>Abstract</B></P> <P>Manipulating the morphology of three-dimensional (3D) cellular structures in both macroscale and microscale is very important in designing a sound absorbing material effective in a broad frequency bandwidth. However, it is extremely difficult to synthesize the hierarchical porous structures having nano- and micro-pores in a self-assembled or controlled manner. Herein, we report an efficient strategy to fabricate the self-aligned and hierarchically porous graphene-polyurethane foams as a sound absorbing material via electrostatic repulsion mechanism of graphene oxide and potassium hydroxide activation. Two unique microscopic cellular structures are designed by controlling the morphology of graphene layers inside the polyurethane backbone using spontaneous self-alignment and stochastic disruptive methods, respectively. The obtained heterogeneous graphene microstructure network provides enhanced mechanically load-bearing ability and significantly improves the sound energy attenuation performance over 312% compared to pristine sound absorber depending on the existence of ordered or disordered graphene lattices, resulting in an efficient pathway for rapidly decaying out acoustic wave energy.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>