Featured Publications

chemicalreviewslogoAtomic force microscopy-based infrared spectroscopy (AFM-IR) is a rapidly emerging technique that provides chemical analysis and compositional mapping with spatial resolution far below conventional optical diffraction limits. AFM-IR works by using the tip of an AFM probe to locally detect thermal expansion in a sample resulting from absorption of infrared radiation. AFM-IR thus can provide the spatial resolution of AFM in combination with the chemical analysis and compositional imaging capabilities of infrared spectroscopy. This article briefly reviews the development and underlying technology of AFM-IR, including recent advances, and then surveys a wide range of applications and investigations using AFM-IR. AFM-IR applications that will be discussed include those in polymers, life sciences, photonics, solar cells, semiconductors, pharmaceuticals, and cultural heritage.


Infrared vibrational nanocrystallography and nanoimaging

Science Advances, 2 (10), e1601006 - E. A. Muller, et al
science-advances-logoMolecular solids and polymers can form low-symmetry crystal structures that exhibit anisotropic electron and ion mobility in engineered devices or biological systems. The distribution of molecular orientation and disorder then controls the macroscopic material response. We demonstrated a new form of optical nanocrystallography that combines scattering-type scanning near-field optical microscopy with both optical antenna and tip-selective infrared vibrational spectroscopy. From the symmetry-selective probing of molecular bond orientation we determined crystalline phases and orientation in aggregates and films of perylenetetracarboxylic dianhydride. Mapping disorder within and between individual nanoscale domains, the correlative hybrid imaging of nanoscale heterogeneity provides insight into defect formation and propagation during growth in functional molecular solids.

ncomms-logoFor the first time, the combination of the complimentary nanoscale imaging techniques, AFM-IR and s-SNOM have been used to investigate the role of chirality in the origins of circular dichroism in 2D nanoscale materials. Chiral molecules are a certain type of molecule that have a non super imposable mirror image. These mirror images of chiral molecules are often called left handed and right handed, and due to the vector nature of light, it can also exist with two forms of handedness, left and right circularly polarized. Fully two-dimensional (2D) metamaterials, also known as metasurfaces, comprised of planar-chiral plasmonic metamolecules that are just nanometres thick, have been shown to exhibit chiral dichroism in transmission (CDT). Theoretical calculations indicate that this surprising effect relies on finite non-radiative (Ohmic) losses of the metasurface. Until now this surprising theoretical prediction has never been experimentally verified because of the challenge of measuring non-radiative loss on the nanoscale. s-SNOM is used to map the optical energy distribution when the structures are exposed to RCP and LCP IR radiation while AFM-IR was then used to detect the drastically different Ohmic heating observed under RCP and LCP radiation. For the first time it has been conclusively established the circular dichroism observed in 2D metasurfaces is attributed to handedness dependent Ohmic heating.

Conducting polymer nanostructures for photocatalysis under visible light

Nature Materials 14, 505–511 - S. Ghosh, et al

nature materialsVisible-light-responsive photocatalysts can directly harvest energy from solar light, offering a desirable way to solve energy and environment issues. Here, we show that one-dimensional poly(diphenylbutadiyne) nanostructures have high photocatalytic activity under visible light without the assistance of sacrificial reagents or precious metal co-catalysts. Transmission electron microscopy and nanoscale infrared characterizations show that polymer nanostructures remain unchanged after many photocatalytic cycles. Our findings may aid in the development of semiconducting-based polymers for applications in self-cleaning surfaces, hydrogen generation and photovoltaics.

naturephotonicslogoThe ability to perform mid-infrared spectroscopy with nanometer spatial resolution is highly desirable for applications in materials and life sciences. At present, scattering near-field scanning optical microscopy is considered to be the most sensitive technique for nanoscale mid-infrared spectroscopy under ambient conditions. We demonstrate that nanoscale mid-infrared spectra can be obtained with comparable or higher sensitivity by detecting mechanical forces exerted by molecules on the atomic force microscope tip on light excitation. This approach results in a simple optical set-up that requires no cryogenically cooled mid-infrared detectors, is easy to align, and is not affected by sample scattering.

logo-nanolettersCH3NH3PbI3-xClx perovskites enable fabrication of highly efficient solar cells. In this work, the photothermal-induced resonance, is leveraged to measure the bandgap of CH3NH3PbI3-xClx films obtained by a multicycle coating process that produces high efficiency solar cells. Because chloride ions modify the perovskite lattice, thereby widening the bandgap, measuring the bandgap locally yields the local chloride content. After a mild annealing the films consist of Cl-rich and Cl-poor phases that upon further annealing evolve into a homogeneous Cl-poorer phase, suggesting that methylammonium-chloride is progressively expelled from the film. Despite the small chloride content, CH3NH3PbI3-xClx films show better thermal stability up to 140 °C with respect to CH3NH3PbI3 films fabricated with the same methodology.

ncomms-logoAmyloids are insoluble protein fibrillar aggregates. The importance of characterizing their aggregation has steadily increased because of their link to human diseases and material science applications. Here we individually characterize the oligomeric and fibrillar species formed along the amyloid aggregation. We describe their secondary structure, monitoring at the nanoscale an α-to-β transition, and couple these studies with an independent measurement of the evolution of their intrinsic stiffness. These results suggest that the aggregation of Josephin proceeds from the monomer state to the formation of spheroidal intermediates with a native structure. Only successively, these intermediates evolve into misfolded aggregates and into the final fibrils.

Fundamental developments in infrared spectroscopic imaging for biomedical applications

Chemical Society Reviews, 2016, 45, 1935-1957 - M. Pilling and P. Gardner
chemicalsocietyrreviewsInfrared chemical imaging is a rapidly emerging field with new advances in instrumentation, data acquisition and data analysis. Relying on purely biochemical signatures rather than contrast from exogenous dyes and stains, infrared chemical imaging has the potential to revolutionize histopathology for improved disease diagnosis. In this review we discuss the recent advances in infrared spectroscopic imaging specifically related to spectral histopathology (SHP) and consider the current state of the field. Finally we consider the practical application of SHP for disease diagnosis and consider potential barriers to clinical translation highlighting current directions and the future outlook.

nature-nanotechnology-logoThe performance and scaling of graphene-based electronics is limited by the quality of contacts between the graphene and metal electrodes. Here, we use atomic force microscopy to measure the temperature distributions at the contacts of working graphene transistors with a spatial resolution of ∼10, allowing us to identify the presence of Joule heating, current crowding and thermoelectric heating and cooling. Our data indicate that thermoelectric effects account for up to one-third of the contact temperature changes, and that current crowding accounts for most of the remainder. Modelling predicts that the role of current crowding will diminish and the role of thermoelectric effects will increase as contacts improve.

logo-nanolettersThe collective oscillation of conduction electrons, responsible for the localized surface plasmon resonances, enables engineering nanomaterials by tuning their optical response from the visible to terahertz as a function of nanostructure size, shape, and environment. In this work, the photothermal-induced resonance (PTIR) technique is applied for the first time to image the dark plasmonic resonance of gold asymmetric split ring resonators (A-SRRs) in the mid-infrared spectral region with nanoscale resolution. Additionally, the PTIR is used to map the local absorption enhancement of poly(methyl methacrylate) coated on A-SRRs. We argue that PTIR nanoscale characterization will facilitate the engineering and application of plasmonic nanomaterials for mid-IR applications.

ncomms-logoSilk protein fibers produced by silkworms and spiders are renowned for their unparalleled mechanical strength and extensibility arising from their high-β-sheet crystal contents. Here, we report on electron-regulated nanoscale polymorphic transitions in silk proteins revealed by near-field infrared imaging and nano-spectroscopy at resolutions approaching the molecular level. The ability to locally probe nanoscale protein structural transitions combined with nanometer-precision electron-beam lithography allows us to finely control the structure of silk proteins in two and three dimensions. Our work paves the way for unlocking essential nanoscopic protein structures and critical conditions for electron-induced conformational transitions, offering new rules to design protein-based nanoarchitectures.

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

Nature Communications. 6:6849 - B.T. O’Callahan, et al
ncomms-logoThe insulator-metal transition (IMT) of vanadium dioxide (VO2) has remained a long-standing challenge in correlated electron physics. Here we reveal inhomogeneous behavior of individual VO2 microcrystals using pump-probe microscopy and nanoimaging. The timescales of the ultrafast IMT vary from 40±8 fs to 200±20 fs with average values similar to results from polycrystalline thin-film studies. In combination with the observed sensitive variations in the thermal nanodomain IMT behavior, this suggests that the IMT is highly susceptible to local changes in doping, defects and strain. Our results suggest an electronic mechanism dominating the photoinduced IMT, but also highlight the difficulty to deduce microscopic mechanisms when the intrinsic material response is unclear.

Advanced MaterialsSolution-processed organometal trihalide perovskite photodetectors show a high photoconductive gain of above 400 across the UV to NIR range at a very low bias of −1 V. The charge traps caused by large concentrations of Pb2+ cations at the top surface of the perovskite film are critical for achieving high gain in these devices via a trapped-hole-induced electron injection mechanism.

Local Nanoscale Heating Modulates Single-Asperity Friction

Nano Letters, 10 (11), pp 4640–4645 - C. Greiner, et al
logo-nanolettersWe demonstrate measurement and control of single-asperity friction by using cantilever probes featuring an in situ solid-state heater. Heating caused friction to increase by a factor of 4 in air at ∼30% relative humidity, but in dry nitrogen friction decreased by ∼40%. Higher velocity reduced friction in ambient with no effect in dry nitrogen. These trends are attributed to thermally assisted formation of capillary bridges between the tip and substrate in air, and thermally assisted sliding in dry nitrogen. Real-time friction measurements while modulating the tip temperature revealed an energy barrier for capillary condensation but with slower kinetics compared to isothermal measurements that we attribute to the distinct thermal environment that occurs when heating in real time. Controlling the presence of this nanoscale capillary and the associated control of friction and adhesion offers new opportunities for tip-based nanomanufacturing.

stretchpvcoverThermal scanning lithography is used to pattern semiconducting nanoribbon-like pentacene structures with ultrahigh spatial resolution onto arbitrary substrates in air. The method allows control of the pentacene crystal growth direction and domain-size distribution. By combining these quasi-one-dimensional nanoribbon-like structures with conductive electrodes and a suitable gate dielectric, functional p-channel transistors are demonstrated.