NO Adsorption on Copper Phthalocyanine Functionalized Graphite

NO dosed on a CuPc monolayer deposited on Au(111) and HOPG is observed by scanning tunneling microscopy. After dosing NO with a supersonic molecular beam source onto CuPc/Au(111), about 7% of CuPc molecules form chemisorbates with NO. Conversely, after dosing onto CuPc/HOPG, only about 0.1% CuPc molecules form chemisorbates with NO, even though the reaction sites appear nearly identical. DFT calculations were employed to elucidate the mechanism which causes the >10× difference in saturation coverage between NO/CuPc/Au(111) and NO/CuPc/HOPG. DFT calculations show NO chemisorption with CuPc/Au(111) induces only negligible perturbation in the density of states (DOS) in Au(111) due to large density of states on Au. Conversely, for NO/CuPc/HOPG, there is a large decrease of DOS in graphene around 1 eV due to NO chemisorption on CuPc/graphene consistent with negative charge transfer from graphene to NO. This DOS perturbation of graphene results in decreased binding energy of NO chemisorption in secondary NO sites, consistent with low saturation coverage. The results suggest that although the saturation coverage of NO chemisorbates is low on CuPc/graphene, the DOS of graphene can be altered by low coverages of adsorbates even onto weakly interacting molecules which chemically functionalize the graphene surface

NO Adsorption on Copper Phthalocyanine Functionalized Graphite

NO dosed on a CuPc monolayer deposited on Au(111) and HOPG is observed by scanning tunneling microscopy. After dosing NO with a supersonic molecular beam source onto CuPc/Au(111), about 7% of CuPc molecules form chemisorbates with NO. Conversely, after dosing onto CuPc/HOPG, only about 0.1% CuPc molecules form chemisorbates with NO, even though the reaction sites appear nearly identical. DFT calculations were employed to elucidate the mechanism which causes the >10× difference in saturation coverage between NO/CuPc/Au(111) and NO/CuPc/HOPG. DFT calculations show NO chemisorption with CuPc/Au(111) induces only negligible perturbation in the density of states (DOS) in Au(111) due to large density of states on Au. Conversely, for NO/CuPc/HOPG, there is a large decrease of DOS in graphene around 1 eV due to NO chemisorption on CuPc/graphene consistent with negative charge transfer from graphene to NO. This DOS perturbation of graphene results in decreased binding energy of NO chemisorption in secondary NO sites, consistent with low saturation coverage. The results suggest that although the saturation coverage of NO chemisorbates is low on CuPc/graphene, the DOS of graphene can be altered by low coverages of adsorbates even onto weakly interacting molecules which chemically functionalize the graphene surface

Air-Stable Spin-Coated Naphthalocyanine Transistors for Enhanced Chemical Vapor Detection

Air-stable organic thin-film transistor (OTFT) sensors fabricated using spin-cast films of 5,9,14,18,23,27,32,36-octabutoxy-2,3-naphthalocyanine (OBNc) demonstrated improved chemical vapor sensitivity and selectivity relative to vacuum-deposited phthalocyanine (H₂Pc) OTFTs. UV–vis spectroscopy data show that annealed spin-cast OBNc films exhibit a red-shift in the OBNc Q-band λ_max which is generally diagnostic of improved π-orbital overlap in phthalocyanine ring systems. Annealed OBNc OTFTs have mobilities of 0.06 cm² V^–1s^–1, low threshold voltages (|<i>V</i>_th| < 1 V), and on/off ratios greater than 10⁶. These air-stable device parameters are utilized for sensing modalities which enhance the sensitivity and selectivity of OBNc OTFTs relative to H₂Pc OTFTs. While both sensors exhibit mobility decreases for all analytes, only OBNc OTFTs exhibit <i>V</i>_th changes for highly polar/nonpolar analytes. The observed mobility decreases for both sensors are consistent with electron donation trends via hydrogen bonding by basic analytes. In contrast, <i>V</i>_th changes for OBNc sensors appear to correlate with the analyte’s octanol–water partition coefficient, consistent with polar molecules stabilizing charge in the organic semiconductor film. The analyte induced <i>V</i>_th changes for OBNc OTFTs can be employed to develop selective multiparameter sensors which can sense analyte stabilized fixed charge in the film

Air-Stable Spin-Coated Naphthalocyanine Transistors for Enhanced Chemical Vapor Detection

Air-stable organic thin-film transistor (OTFT) sensors fabricated using spin-cast films of 5,9,14,18,23,27,32,36-octabutoxy-2,3-naphthalocyanine (OBNc) demonstrated improved chemical vapor sensitivity and selectivity relative to vacuum-deposited phthalocyanine (H₂Pc) OTFTs. UV–vis spectroscopy data show that annealed spin-cast OBNc films exhibit a red-shift in the OBNc Q-band λ_max which is generally diagnostic of improved π-orbital overlap in phthalocyanine ring systems. Annealed OBNc OTFTs have mobilities of 0.06 cm² V^–1s^–1, low threshold voltages (|<i>V</i>_th| < 1 V), and on/off ratios greater than 10⁶. These air-stable device parameters are utilized for sensing modalities which enhance the sensitivity and selectivity of OBNc OTFTs relative to H₂Pc OTFTs. While both sensors exhibit mobility decreases for all analytes, only OBNc OTFTs exhibit <i>V</i>_th changes for highly polar/nonpolar analytes. The observed mobility decreases for both sensors are consistent with electron donation trends via hydrogen bonding by basic analytes. In contrast, <i>V</i>_th changes for OBNc sensors appear to correlate with the analyte’s octanol–water partition coefficient, consistent with polar molecules stabilizing charge in the organic semiconductor film. The analyte induced <i>V</i>_th changes for OBNc OTFTs can be employed to develop selective multiparameter sensors which can sense analyte stabilized fixed charge in the film

Dual Passivation of Intrinsic Defects at the Compound Semiconductor/Oxide Interface Using an Oxidant and a Reductant

Studies have shown that metal oxide semiconductor field-effect transistors fabricated utilizing compound semiconductors as the channel are limited in their electrical performance. This is attributed to imperfections at the semiconductor/oxide interface which cause electronic trap states, resulting in inefficient modulation of the Fermi level. The physical origin of these states is still debated mainly because of the difficulty in assigning a particular electronic state to a specific physical defect. To gain insight into the exact source of the electronic trap states, density functional theory was employed to model the intrinsic physical defects on the InGaAs (2 × 4) surface and to model the effective passivation of these defects by utilizing both an oxidant and a reductant to eliminate metallic bonds and dangling-bond-induced strain at the interface. Scanning tunneling microscopy and spectroscopy were employed to experimentally determine the physical and electronic defects and to verify the effectiveness of dual passivation with an oxidant and a reductant. While subsurface chemisorption of oxidants on compound semiconductor substrates can be detrimental, it has been shown theoretically and experimentally that oxidants are critical to removing metallic defects at oxide/compound semiconductor interfaces present in nanoscale channels, oxides, and other nanostructures

Grazing Incidence Cross-Sectioning of Thin-Film Solar Cells via Cryogenic Focused Ion Beam: A Case Study on CIGSe

Cryogenic focused ion beam (Cryo-FIB) milling at near-grazing angles is employed to fabricate cross-sections on thin Cu­(In,Ga)­Se₂ with >8x expansion in thickness. Kelvin probe force microscopy (KPFM) on sloped cross sections showed reduction in grain boundaries potential deeper into the film. Cryo Fib-KPFM enabled the first determination of the electronic structure of the Mo/CIGSe back contact, where a sub 100 nm thick MoSe_<i>y</i> assists hole extraction due to 45 meV higher work function. This demonstrates that CryoFIB-KPFM combination can reveal new targets of opportunity for improvement in thin-films photovoltaics such as high-work-function contacts to facilitate hole extraction through the back interface of CIGS

Growth Mode Transition from Monolayer by Monolayer to Bilayer by Bilayer in Molecularly Flat Titanyl Phthalocyanine Film

To avoid defects associated with inhomogeneous crystallites and uneven morphology that degrade organic device performance, the deposition of ultraflat and homogeneous crystalline organic active layers is required. The growth mode transition of organic semiconducting titanyl phthalocyanine (TiOPc) molecule from monolayer-by-monolayer to bilayer-by-bilayer can be observed on highly ordered pyrolytic graphic (HOPG), while maintaining large and molecularly flat domains. The first monolayer of TiOPc lies flat on HOPG with a ∼98% face-up orientation. However, as the thickness of the TiOPc increases to over 15 monolayers (ML), the growth mode transitions to bilayer-by-bilayer with the repeated stacking of bilayers (BL), each of which has face-to-face pairs. Density functional theory calculations reveal that the increasing of thickness induces weakening of the substrate effect on the deposited TiOPc layers, resulting in the growth mode transition to BL-by-BL. The asymmetric stacking provides the driving force to maintain nearly constant surface order during growth, allowing precise, subnanometer thickness control and large domain growth

Atomic Imaging of the Irreversible Sensing Mechanism of NO₂ Adsorption on Copper Phthalocyanine

Ambient NO₂ adsorption onto copper­(II) phthalocyanine (CuPc) monolayers is observed using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) to elucidate the molecular sensing mechanism in CuPc chemical vapor sensors. For low doses (1 ppm for 5 min) of NO₂ at ambient temperatures, isolated chemisorption sites on the CuPc metal centers are observed in STM images. These chemisorbates almost completely desorb from the CuPc monolayer after annealing at 100 °C for 30 min. Conversely, for high NO₂ doses (10 ppm for 5 min), the NO₂ induces a fracture of the CuPc domains. This domain fracture can only be reversed by annealing above 150 °C, which is consistent with dissociative chemisorption into NO and atomic O accompanied by surface restructuring. This high stability implies that the domain fracture results from tightly bound adsorbates, such as atomic O. Existence of atomic O on or under the CuPc layer, which results in domain fracture, is revealed by XPS analysis and ozone-dosing experiments. The observed CuPc domain fracturing is consistent with a mechanism for the dosimetric sensing of NO₂ and other reactive gases by CuPc organic thin film transistors (OTFTs)

Nanoscale Characterization of Back Surfaces and Interfaces in Thin-Film Kesterite Solar Cells

Combinations of sub 1 μm absorber films with high-work-function back surface contact layers are expected to induce large enough internal fields to overcome adverse effects of bulk defects on thin-film photovoltaic performance, particularly in earth-abundant kesterites. However, there are numerous experimental challenges involving back surface engineering, which includes exfoliation, thinning, and contact layer optimization. In the present study, a unique combination of nanocharacterization tools, including nano-Auger, Kelvin probe force microscopy (KPFM), and cryogenic focused ion beam measurements, are employed to gauge the possibility of surface potential modification in the absorber back surface via direct deposition of high-work-function metal oxides on exfoliated surfaces. Nano-Auger measurements showed large compositional nonuniformities on the exfoliated surfaces, which can be minimized by a brief bromine–methanol etching step. Cross-sectional nano-Auger and KPFM measurements on Au/MoO₃/Cu₂­ZnSn­(S,Se)₄ (CZTSSe) showed an upward band bending as large as 400 meV within the CZTSSe layer, consistent with the high work function of MoO₃, despite Au incorporation into the oxide layer. Density functional theory simulations of the atomic structure for bulk amorphous MoO₃ demonstrated the presence of large voids within MoO₃ enabling Au in-diffusion. With a less diffusive metal electrode such as Pt or Pd, upward band bending beyond this level is expected to be achieved

Ultrasound Responsive Macrophase-Segregated Microcomposite Films for <i>in Vivo</i> Biosensing

Ultrasound imaging is a safe, low-cost, and <i>in situ</i> method for detecting <i>in vivo</i> medical devices. A poly­(methyl-2-cyanoacrylate) film containing 2 μm boron-doped, calcined, porous silica microshells was developed as an ultrasound imaging marker for multiple medical devices. A macrophase separation drove the gas-filled porous silica microshells to the top surface of the polymer film by controlled curing of the cyanoacrylate glue and the amount of microshell loading. A thin film of polymer blocked the wall pores of the microshells to seal air in their hollow core, which served as an ultrasound contrast agent. The ultrasound activity disappeared when curing conditions were modified to prevent the macrophase segregation. Phase segregated films were attached to multiple surgical tools and needles and gave strong color Doppler signals <i>in vitro</i> and <i>in vivo</i> with the use of a clinical ultrasound imaging instrument. Postprocessing of the simultaneous color Doppler and B-mode images can be used for autonomous identification of implanted surgical items by correlating the two images. The thin films were also hydrophobic, thereby extending the lifetime of ultrasound signals to hours of imaging in tissues by preventing liquid penetration. This technology can be used as a coating to guide the placement of implantable medical devices or used to image and help remove retained surgical items