Characterizing self-assembly and deposition behavior of nanoparticles in inkjet-printed evaporating droplets

Thokchom, Ashish Kumar,Zhou, Qitao,Kim, Dong-Joo,Ha, Dogyeong,Kim, Taesung Elsevier Sequoia 2017 Sensors and actuators. B Chemical Vol.252 No.-

<P><B>Abstract</B></P> <P>The self-assembly and deposition mechanisms of nanoparticles in droplets on a substrate are of significant importance in many inkjet printing-based industrial applications such as microelectronics, display systems, and paint manufacturing. However, a comprehensive investigation into the velocity field of fluid and its accompanying particle transport behavior in injected droplets undergoing immediate evaporation has not been conducted. In this study, we describe the underlying mechanisms of the self-assembly and deposition behavior of nanoparticles in inkjet-printed, evaporating droplets by visualizing the internal fluid flows. We additionally characterize the relationship between the internal fluid flows and nanoparticle patterns by changing not only the wettability and temperature of the substrate, but also the chemical composition of nanoparticle suspensions. We verify that Marangoni flow generated on a hydrophobic PDMS substrate with a contact angle (CA) of >90° helps the formation of dome-shaped nanoparticle structures, while radially outward flow generated on a hydrophilic glass substrate with a CA of <10° produces either mono-layered and flat, or ring-shaped nanoparticle structures, depending on the number density of the suspension. The presented characterization results provide not only valuable mechanistic insights, but also practical guidelines for inkjet printing-based nanoparticle applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Marangoni flow on a hydrophobic substrate helps the formation of dome-shaped nanoparticle structures. </LI> <LI> Radially outward flow on a hydrophilic substrate produces either flat and mono-layered or ring-shaped nanoparticle structures. </LI> <LI> The nanoparticle structures on a hydrophilic substrate depend on the number density of the suspension. </LI> <LI> The substrate temperature affects the self-assembly and deposition mechanism of nanoparticles in evaporating droplets. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>This study describes the mechanism of the self-assembly and deposition of nanoparticles in droplets that are inkjet-injected and immediately evaporate on various substrates. The resulting nanostructures and patterns directly affect structural colors showing high potential for anti-counterfeit applications.</P> <P>[DISPLAY OMISSION]</P>

Visualization of Microfluid Flow in Evaporating Droplets for Self-Assembly Control of Nanoparticles

Ashish Kumar Thokchom,Qitao Zhou,Jungyu Park,Dogyeong Ha,Taesung Kim 한국가시화정보학회 2017 한국가시화정보학회 학술발표대회 논문집 Vol.2017 No.11

Covert and Overt transformation of Structural Colors Formed by Inkjet Printing of Nanoparticle Suspension for Durable and Flexible Anti-counterfeit Applications

이경훈,Ashish Kumar Thokchom,Qitao Zhou,하도경,박준규,박정열,김태성 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.5

Inkjet-printed Ag micro-/nanostructure clusters on Cu substrates for in-situ pre-concentration and surface-enhanced Raman scattering

Zhou, Qitao,Thokchom, Ashish Kumar,Kim, Dong-Joo,Kim, Taesung Elsevier Sequoia 2017 Sensors and actuators. B Chemical Vol.243 No.-

<P><B>Abstract</B></P> <P>Effective surface-enhanced Raman scattering (SERS) detection requires substrates that are typically fabricated using expensive, low-throughput and time-consuming micro-/nanofabrication processes such as photolithography, electron-beam lithography and template- assisted methods Here, a novel micro-/nanofabrication technique for fabricating SERS substrates with hydrophobicity gradients is demonstrated. An inkjet printer enables injecting an AgNO<SUB>3</SUB> solution onto a thiol-functionalized superhydrophobic Cu surface, upon which Ag micro/nanostructures are generated via replacement reactions in the droplet-injected areas. When a mixed solution of target analytes and Au-nanoparticles (Au-NPs) are placed on this substrate, the contact area decreases over time due to the evaporation of the solution and the hydrophobicity of the substrate. As a result, the analyte molecules and Au-NPs are delivered to the Ag micro-/nanostructure clusters, upon which the analyte and Au-NPs are simply and easily concentrated in situ. With the cooperation of Ag nanoplates and Au-NPs, two antibiotics at very low concentrations (e.g., 100pM 6-aminopenicillanic acid and 50pM penicillin G sodium) were successfully detected, confirming the higher SERS activity than that of Ag-nanoplate-assembled nanotube arrays or an Ag-NPs decorated graphene electrophoretic pre-concentration device. Hence, this rapid design-to-prototype method for substrates with adjustable wetting properties can be very useful for a SERS platform to detect various target analytes in biosamples.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A novel inkjet printing technique for fabricating SERS substrates is presented. </LI> <LI> In-situ fabrication of noble metal nanoarrays for high density SERS “hot spots” and wettability gradients for analyte pre-concentration. </LI> <LI> Highly sensitive SERS detection of antibiotics at very low concentrations below 100pM. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Surface-enhanced Raman scattering is a promising technology for biosensing but the challenge of creating practical substrates remains. This study presents a novel inkjet printing technique for fabricating (SERS) substrates that offer both in-situ fabrication of the noble metal nanoarrays, and thus high density SERS “hot spots” but also pre-concentration of the target analyte.</P> <P>[DISPLAY OMISSION]</P>

Experimental and computational study on flow over stepped spillway

Bhaskar Jyoti Medhi,Anugrah Singh,Ashish Kumar Thokchom,Sadhan Mahapatra 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.5

The flow over a stepped spillway has complex nature, and its characteristics are remarkably different from other kinds of spillways. This study conducts experimental investigations and numerical simulations on the flow behavior (velocity, concentration profile) and macroscopic features (interface position and self-aeration) of water and neutrally buoyant suspension of non-colloidal particles in a stepped spillway with uniform steps. The development of nappe, transition, and skimming flow regimes is experimentally investigated by using a flow visualization technique. The inception point related to air entrainments is identified in the experimental study. The inception point usually moves downstream and increases the length of the non-aerated region with the increase of flow rate. Results of numerical and experimental studies indicate that a vortex is formed in the triangular cavity below the pseudo-bottom line (imaginary line joining two adjacent step edges) in the stepped channel. This vortex rotates in a clockwise direction for a short time period and returns to the main flow to move downward in the channel. The velocity vector map from numerical simulation predicts the maximum velocity in the middle portion of the spillway, that is, near the pseudo-bottom line. A volume of fluid model coupled with a standard k–e turbulence model is used in the CFD simulations to predict the location of the air–water air-suspension interface. The results are compared with experimental measurements. The calculated interface position agrees well with the experimental measurements. The migration and transport of particles are evaluated based on a diffusive flux model of shear induced particle migration. The contour map for velocity and particle concentration shows a remarkable increase in particle concentration near the air-suspension interface.

Nanochannel-Assisted Perovskite Nanowires: From Growth Mechanisms to Photodetector Applications

Zhou, Qitao,Park, Jun Gyu,Nie, Riming,Thokchom, Ashish Kumar,Ha, Dogyeong,Pan, Jing,Seok, Sang Il,Kim, Taesung American Chemical Society 2018 ACS NANO Vol.12 No.8

<P>Growing interest in hybrid organic-inorganic lead halide perovskites has led to the development of various perovskite nanowires (NWs), which have potential use in a wide range of applications, including lasers, photodetectors, and light-emitting diodes (LEDs). However, existing nanofabrication approaches lack the ability to control the number, location, orientation, and properties of perovskite NWs. Their growth mechanism also remains elusive. Here, we demonstrate a micro/nanofluidic fabrication technique (MNFFT) enabling both precise control and <I>in situ</I> monitoring of the growth of perovskite NWs. The initial nucleation point and subsequent growth path of a methylammonium lead iodide-dimethylformamide (MAPbI<SUB>3</SUB>·DMF) NW array can be guided by a nanochannel. <I>In situ</I> UV-vis absorption spectra are measured in real time, permitting the study of the growth mechanism of the DMF-mediated crystallization of MAPbI<SUB>3</SUB>. As an example of an application of the MNFFT, we demonstrate a highly sensitive MAPbI<SUB>3</SUB>-NW-based photodetector on both solid and flexible substrates, showing the potential of the MNFFT for low-cost, large-scale, highly efficient, and flexible optoelectronic applications.</P> [FIG OMISSION]</BR>

Transparent-flexible-multimodal triboelectric nanogenerators for mechanical energy harvesting and self-powered sensor applications

Zhou, Qitao,Park, Jun Gyu,Kim, Kyeong Nam,Thokchom, Ashish Kumar,Bae, Juyeol,Baik, Jeong Min,Kim, Taesung Elsevier 2018 Nano energy Vol.48 No.-

<P><B>Abstract</B></P> <P>Triboelectric nanogenerators (TENGs) harvest and convert mechanical energy to electrical energy. TENGs that are transparent and flexible can be applied to various (opto-)electronic devices supporting finger- or pen-based touchscreen inputs. This paper presents a transparent, flexible TENG that harvests mechanical tapping energy (typically discarded) by simple placement on touchscreen devices. The developed TENG consists of flexible and transparent conducting electrodes (FTCE) with high transmittance (> 93%) and low sheet resistance (18.5 Ω/sq), and transparent 3D-hierarchical polydimethylsiloxane (PDMS) with porous pyramid-patterns. In this study, the developed TENG directly powered eight light-emitting diodes (LEDs) by harvesting the mechanical energy produced by tapping with a touch pen while playing a smartphone game. We also used the transparent TENG as a transparent single-electrode-based, self-powered raindrop detection sensor on a window for a smart home. Our results indicate that the proposed TENG can be used not only as an effective mechanical energy harvester for transparent, flexible, and next-generation optoelectronics devices but also as a self-powered sensor for future Internet-of-Things applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Flexible transparent conducting electrodes (FTCEs) are fabricated by an inkjet printer. </LI> <LI> A PDMS interlayer with 3D micro/-nanostructures is prepared by particle lithography. </LI> <LI> A transparent and flexible TENG is fabricated by the FTCEs and PDMS interlayers. </LI> <LI> The TENG harvests the mechanical energy produced when tapping electronic devices. </LI> <LI> The TENG works as single-electrode-based self-powered raindrop sensors. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>