Nicholas A. Charipar

Low-temperature plasma probe for ambient desorption ionization.

A low-temperature plasma (LTP) probe has been developed for ambient desorption ionization. An ac electric field is used to induce a dielectric barrier discharge through use of a specially designed electrode configuration. The low-temperature plasma is extracted from the probe where it interacts directly with the sample being analyzed, desorbing and ionizing surface molecules in the ambient environment. This allows experiments to be performed without damage to the sample or underlying substrate and, in the case of biological analysis on skin surfaces, without electrical shock or perceptible heating. Positive or negative ions are produced from a wide range of chemical compounds in the pure stateand as mixtures in the gaseous, solution, or condensed phases, using He, Ar, N2, or ambient air as the discharge gas. Limited fragmentation occurs, although it is greater in the cases of the molecular than the atomic discharge gases. The effectiveness of the LTP probe has been demonstrated by recording characteristic mass spectra and tandem mass spectra of samples containing hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT) from poly(tetrafluoroethylene) (PTFE) surfaces where limits of detection are as low as 5 pg. Other performance characteristics, when using a commercial ion trap mass spectrometer, include 3-4 orders of magnitude linear dynamic range in favorable cases. Demonstration applications include direct analysis of cocaine from human skin, determination of active ingredients directly in drug tablets, and analysis of toxic and therapeutic compounds in complex biological samples. Ionization of chemicals directly from bulk aqueous solution has been demonstrated, where limits of detection are as low as 1 ppb. Large surface area sampling and control of fragmentation by a simple adjustment of the electrode configuration during operation are other demonstrated characteristics of the method.

Detection of explosives and related compounds by low-temperature plasma ambient ionization mass spectrometry.

Detection of explosives is important for public safety. A recently developed low-temperature plasma (LTP) probe for desorption and ionization of samples in the ambient environment ( Anal. Chem. 2008 , 80 , 9097 ) is applied in a comprehensive evaluation of analytical performance for rapid detection of 13 explosives and explosives-related compounds. The selected chemicals [pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT), cyclo-1,3,5-trimethylenetrinitramine (RDX), tetryl, cyclo-1,3,5,7-tetramethylenetetranitrate (HMX), hexamethylene triperoxide diamine (HMTD), 2,4-dinitrotoluene, 1,3-dinitrobenzene, 1,3,5-trinitrobenzene, 2-amino-4,6-dinitrotoluene, 4-amino-2,6-dinitrotoluene, 2,6-dinitrotoluene, and 4-nitrotoluene) were tested at levels in the range 1 pg-10 ng. Most showed remarkable sensitivity in the negative-ion mode, yielding limits of detection in the low picogram range, particularly when analyzed from a glass substrate heated to 120 °C. Ions typically formed from these molecules (M) by LTP include [M + NO(2)](-), [M](-), and [M - NO(2)](-). The LTP-mass spectrometry methodology displayed a linear signal response over three orders of magnitude of analyte amount for the studied explosives. In addition, the effects of synthetic matrices and different types of surfaces were evaluated. The data obtained demonstrate that LTP-MS allows detection of ultratrace amounts of explosives and confirmation of their identity. Tandem mass spectrometry (MS/MS) was used to confirm the presence of selected explosives at low levels; for example, TNT was confirmed at absolute levels as low as 0.6 pg. Linearity and intra- and interday precision were also evaluated, yielding results that demonstrate the potential usefulness and ruggedness of LTP-MS for the detection of explosives of different classes. The use of ion/molecule reactions to form adducts with particular explosives such as RDX and HMX was shown to enhance the selectivity and specificity. This was accomplished by merging the discharge gas with an appropriate reagent headspace vapor (e.g., from a 0.2% trifluoracetic acid solution).

Three‐Dimensional Printing of Interconnects by Laser Direct‐Write of Silver Nanopastes

Conventional interconnect technologies are facing increasing challenges as dramatic advances in features and performance are made in microelectronics systems. Wire bonding technology is the most common fi rst-level chip interconnection method used throughout the electronics industry today. As semiconductor devices continue to shrink in feature size and increase in functionality, the capability to bond smaller pads with ultra-fi ne pitch ( < 50 μ m) and reduced interconnect height ( < 60 μ m) is required for many microelectronic, optoelectronic and bioelectronic devices. [ 1–5 ] These dimensions lie below the current limits of wire bonding technology. In addition, the implementation of hybrid structures in printed electronics requires interconnect technologies compatible with dissimilar materials and fl exible plastic surfaces at low processing temperatures. [ 6 , 7 ] Therefore, alternative interconnection technologies are required to solve challenges presented by resolution advances and compatibility requirements in the development of new microelectronic structures and applications. Direct-write (DW) is a family of non-lithographic approaches that allow rapid and low-cost fabrication for printed electronics, photonic materials, sensor, micropower sources and biological applications. [ 8–15 ] Most direct-write techniques, such as inkjet printing, [ 8 ] are constrained to two-dimensional (2D) patterning. Extrusion-based direct ink writing technologies [ 16–18 ] are able to produce spanning structures, however, the cross-section geometry of the extrudates is usually invariable during the printing process and close contact with the substrate or previously deposited layers is required to assemble the new layers. To date, there have been few reports of 3D direct-write processes that combine non-contact printing and assembly, variable geometries of the building blocks during the printing and fi ne 3D resolution. Such a freeform direct-write process could provide a powerful tool for fabricating interconnects with ultra-fi ne pitch bonding capability. Many applications, such as delicate electronic devices, organic electronics and MEMS fabrication, would also benefi t from them. Here we report a novel non-contact three-dimensional printing and assembly process based on laser-induced forward transfer (LIFT) of high concentration silver nanopastes, also called laser direct-write (LDW). [ 15 ]

Direct olive oil analysis by low-temperature plasma (LTP) ambient ionization mass spectrometry

A fast, reagentless, and direct method is presented for the mass spectrometric analysis of olive oil without any sample pretreatment whatsoever. An ambient ionization technique, the low-temperature plasma (LTP) probe, based on dielectric barrier discharge, is used to detect both minor and trace components (free fatty acids, phenolics and volatiles) in raw untreated olive oil. The method allows the measurement of free fatty acids (the main quality control parameter used to grade olive oil according to quality classes), selected bioactive phenolic compounds, and volatiles. The advantages and limitations of the direct analysis of extremely complex mixtures by the ambient ionization/tandem mass spectrometry combination are discussed and illustrated. The data presage the possible large-scale application of direct mass spectrometric analysis methods in the characterization of olive oil and other foodstuffs.

Fabrication of terahertz metamaterials by laser printing

A laser printing technique was used to fabricate split-ring resonators (SRRs) on Si substrates for terahertz (THz) metamaterials and their resonance behavior evaluated by THz time-domain spectroscopy. The laser-printed Ag SRRs exhibited sharp edge definition and excellent thickness uniformity, which resulted in an electromagnetic response similar to that from identical Au SRR structures prepared by conventional photolithography. These results demonstrate that laser printing is a practical alternative to conventional photolithography for fabricating metamaterial structures at terahertz frequencies, since it allows their design to be easily modified and optimized.

Laser 3D micro-manufacturing

Laser-based materials processing techniques are gaining widespread use in micro-manufacturing applications. The use of laser microfabrication techniques enables the processing of micro- and nanostructures from a wide range of materials and geometries without the need for masking and etching steps commonly associated with photolithography. This review aims to describe the broad applications space covered by laser-based micro- and nanoprocessing techniques and the benefits offered by the use of lasers in micro-manufacturing processes. Given their non-lithographic nature, these processes are also referred to as laser direct-write and constitute some of the earliest demonstrations of 3D printing or additive manufacturing at the microscale. As this review will show, the use of lasers enables precise control of the various types of processing steps—from subtractive to additive—over a wide range of scales with an extensive materials palette. Overall, laser-based direct-write techniques offer multiple modes of operation including the removal (via ablative processes) and addition (via photopolymerization or printing) of most classes of materials using the same equipment in many cases. The versatility provided by these multi-function, multi-material and multi-scale laser micro-manufacturing processes cannot be matched by photolithography nor with other direct-write microfabrication techniques and offer unique opportunities for current and future 3D micro-manufacturing applications.

Strain Effects in Epitaxial VO2 Thin Films on Columnar Buffer-Layer TiO2/Al2O3 Virtual Substrates

Epitaxial VO2/TiO2 thin film heterostructures were grown on (100) (m-cut) Al2O3 substrates via pulsed laser deposition. We have demonstrated the ability to reduce the semiconductor-metal transition (SMT) temperature of VO2 to ∼44 °C while retaining a 4 order of magnitude SMT using the TiO2 buffer layer. A combination of electrical transport and X-ray diffraction reciprocal space mapping studies help examine the specific strain states of VO2/TiO2/Al2O3 heterostructures as a function of TiO2 film growth temperatures. Atomic force microscopy and transmission electron microscopy analyses show that the columnar microstructure present in TiO2 buffer films is responsible for the partially strained VO2 film behavior and subsequently favorable transport characteristics with a lower SMT temperature. Such findings are of crucial importance for both the technological implementation of the VO2 system, where reduction of its SMT temperature is widely sought, as well as the broader complex oxide community, where greater understanding of the evolution of microstructure, strain, and functional properties is a high priority.

Applications of laser direct-write for embedding microelectronics

The use of direct-write techniques might revolutionize the way microelectronic devices such as interconnects, passives, ICs, antennas, sensors and power sources are designed and fabricated. The Naval Research Laboratory has developed a laser-based microfabrication process for direct-writing the materials and components required for the assembly and interconnection of the above devices. This laser direct-write (LDW) technique is capable of operating in subtractive, additive, and transfer mode. In subtractive mode, the system operates as a laser micromachining workstation capable of achieving precise depth and surface roughness control. In additive mode, the system utilizes a laser-forward transfer process for the deposition of metals, oxides, polymers and composites under ambient conditions onto virtually any type of surface, thus functioning as a laser printer for patterns of electronic materials. Furthermore, in transfer mode, the system is capable of transferring individual devices, such as semiconductor bare die or surface mount devices, inside a trench or recess in a substrate, thus performing the same function of the pick-and-place machines used in circuit board manufacture. The use of this technique is ideally suited for the rapid prototyping of embedded microelectronic components and systems while allowing the overall circuit design and layout to be easily modified or adapted to any specific application or form factor. This paper describes the laser direct-write process as applied to the forward transfer of microelectronic devices.

Electrowetting Displays Utilizing Bistable, Multi-Color Pixels Via Laser Processing

Electronic paper, or e-Paper, for use in displays has seen rapid growth in the past decade because of its potential as an alternative to traditional transmissive displays. Offering several critical advantages over current display technologies, including high contrast in direct sunlight, wide viewing angles, and compatibility with flexible substrate processing, electrowetting displays (EWDs) have made it to the forefront of e-Paper research and development efforts. Here, we describe a new design for the fabrication of multi-color, bistable electrowetting displays. Using a laser-based process to pattern an in-plane electrode design, liquid can be manipulated out-of-plane. This process relies on electromechanical pressure forcing water in and out of channels, causing colored oil to be displaced. When voltage is removed, the oil remains in its current position, resulting in bistability. We have demonstrated multi-color, bistable pixels that maintain their state at V = 0 for several days, which drastically reduces the power required to drive the display.

Broadband terahertz generation using the semiconductor-metal transition in VO2

We report the design, fabrication, and characterization of broadband terahertz emitters based on the semiconductor-metal transition in thin film VO2 (vanadium dioxide). With the appropriate geometry, picosecond electrical pulses are generated by illuminating 120 nm thick VO2 with 280 fs pulses from a femtosecond laser. These ultrafast electrical pulses are used to drive a simple dipole antenna, generating broadband terahertz radiation.