Atomic Force Microscopy Journal Club

As part of our work in atomic force microscopy, we regularly come across great research articles. Members of our Atomic Force Microscopy Journal Club receive brief reviews of select papers, collected below. If you would like to be added to our email list for these reviews, please email us at productinfo@bruker.com.

Materials Research

INFLUENCE OF THE POLYTETRAFLUORETHLYLENE CONTENT ON THE PERFORMANCE OF HIGH-TEMPERATURE POLYMER ELECTROLYTE MEMBRANE FUEL CELL ELECTRODES
Florian Mack, Tobias Morawietz, Renate Hiesgen, Dominik Kramer, Viktor Gogel, Roswitha Zeis
Int. J. Hydrogen Energy, 2016, 41(18), pp 7475-7483 8
DOI: 10.1016/j.ijhydene.2016.02.156
(view article: http://www.sciencedirect.com/science/article/pii/S0360319916310035)


In high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), the catalyst layer contains both the carbon-supported Pt catalyst and a binder for the mechanical stability. Polytetrafluoroethylene (PTFE) is often used as the binder materials for the high operating temperature. The binder is believed to influence the performance of HT-PEMFCs while it remains poorly investigated on the microscopic scale about the binder effects on key parameters such as ionic conductivity, transport properties, and catalyst utilization. In this work, to visualize the nanostructures of the HT-PEMFC catalyst layer and to characterize the local nanomechanical and nanoelectrical properties, PeakForce Tunneling AFM (PF-TUNA) was used. PF-TUNA is an advanced conductive AFM technique that allows simultaneously capturing correlated topographic, mechanical and electrical information. The authors showed that with high PTFE content the catalyst particles was increasingly covered, resulting in decreased electrochemical active area and reduced performance. On the other hand, over reduction of PTFE content also has negative impacts on the cell performance. The microscopic studies assisted the authors to optimize the content of PFTE, which is 5%, for the highest performance.

SOLUTION-PROCESSABLE CARBON NANOELECTRODES FOR SINGLE-MOLECULE INVESTIGATION
Jingyuan Zhu, Joseph McMorrow, Rachel Crespo-Otero, Geyou Ao, Ming Zheng, William P. Gillin, and Matteo Palma
Journal of the American Chemical Society, 2016, 138 (9), 2905–2908
DOI: 10.1021/jacs.5b12086
(view full article: http://pubs.acs.org/doi/abs/10.1021/jacs.5b12086)


All-carbon based molecular scale electronics is desired for shrinking electrical circuits due to the intrinsic nanosize of CNTs/graphene and the minimized electronic mismatch at the junctions. In this work, solution-based self-assembly of DNA-wrapped SWCNTs with oligophenyls linkers were employed to form the molecular transport junctions (MTJs). PeakForce imaging with unique capabilities in high-resolution mapping of fragile and delicate samples was used to characterize both the topography and length distribution of these MTJs. More importantly, PF-TUNA was applied for electrical conductivity imaging. The authors found that the conductivity within each segment of SWCNTs along the MTJs is homogeneous. The current amplitude depends on two factors. One is the number of molecular bridges between the segment and the contact electrode from which the authors measured the resistance of each individual p-phenylenediamine-SWCNT junction to be ~3.1 MΩ. The other is the number of phenyl rings of the linkers, which shows that the conductance decays exponentially with molecular length. This work is the first example of solution-processable carbon-based MTJs and will be employed to produce a variety of solution-processable nanoelectronic devices.

DEMONSTRATION OF ASYMMETRIC ELECTRON CONDUCTION IN PSEUDOSYMMETRICAL PHOTOSYNTHETIC REACTION CENTRE PROTEINS IN AN ELECTRICAL CIRCUIT
Muhammad Kamran, Vincent M. Friebe, Juan D. Delgado, Thijs J. Aartsma,
Raoul N. Frese and Michael R. Jones
Nature Communications (2015), 6, 6530.
DOI: 10.1038/ncomms7530
(view full article: http://www.nature.com/ncomms/2015/150309/ncomms7530/full/ncomms7530.html)


In billions of years of evolution, Mother Nature has created a sophisticated photosynthetic system supporting the living system on Earth. The two reaction centers, photosystem I and photosystem II, are blue prints for a variety of practical devices, such as biomolecular electronics, artificial leaves and others. Symmetry plays a key role in these reaction centers. Structural asymmetries in these reaction centers result in 1) a diode-like photochemical charge separation where 2) only one of the asymmetric cofactor wires is photoconductive. In the present work, Kamran et al use conductive AFM to study the diode-like behavior, addressing the asymmetric electron conduction in the two cofactor wires of Rba. Sphaeroides reaction centers. Their study reveals that under a suitably oriented bias only one of wires is able to conduct unidirectional current. Thus the complex photochemical stimulation process normally used in nature can be replaced by a simple external bias for control of the electron flow in this reaction center.

ATMOSPHERIC AND AQUEOUS DEPOSITION OF POLYCRYSTALLINE METAL OXIDES USING MIST-CVD FOR HIGHLY EFFICIENT INVERTED POLYMER SOLAR CELLS
Xiaodan Zhu, Toshiyuki Kawaharamura, Adam Z. Stieg, Chandan Biswas,Lu Li, Zhu Ma, Mark A. Zurbuchen, Qibing Pei,and Kang L. Wang
Nano Letter (2015), 15, 4948
DOI: 10.1021/acs.nanolett.5b01157
(view full article: http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b01157 )


In this work, the authors demonstrate the growth of high-quality polycrystalline metal oxide thin films through the readily scalable method of mist chemical vapor deposition (CVD). Use of the resultant films in inverted polymer solar cells is shown to yield high device performance. Zhu et al use AFM topography images to characterize the surface morphology and roughness, and understand the film growth mechanism. Additional conductive AFM (CAFM) measurements give insight into the local electronic properties and film coverage. As the insulating metal oxide film is deposited onto a conductive substrate, pinholes open conductive channels, making them readily apparent as hot-spots in CAFM current maps, thus enabling the study of film coverage as a function of growth parameters. While demonstrated in the present work for ZnO, the showcased film growth approach may be transferrable to other metal oxides, potentially opening the door to cost effective manufacturing of thin film optoelectronics.

QUANTUM MECHANICAL RIPPLING OF A MOS2 MONOLAYER CONTROLLED BY INTERLAYER BILAYER COUPLING
Yi Zheng, Jianyi Chen, M.-F. Ng, Hai Xu, Yan Peng Liu, Ang Li, Sean J. O’Shea, T. Dumitrică, and Kian Ping Loh
Physical Review Letters (2015)
doi: 10.1103/PhysRevLett.114.065501
(view full article: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.065501 )


MoS2 belongs to the ‘hot’ class of 2D-materials. Its 2D layers resemble the graphene structure and possess some of graphene’s exceptional qualities such as mechanical strength. Interlayer coupling results in distinctly different behavior of 2D and bulk material, but its effect on mechanical properties is not well understood. Here, the authors grow MoS2 bilayers on monolayers in different stacking symmetries to study interlayer coupling effects on strain. AFM imaging with pN force control resolves 1D nanoripple arrays in the monolayer, but no rippling in bilayers. This behavior is explained by biaxial strain caused by the interlayer edge-to-basal plane coupling. Hence, quantum mechanics determines the nanoscale rippling, not classical mechanics, a fact that may be common to all 2D materials and hence might impact their electronic and strain engineering.

INTERFACE ENGINEERING OF THE PHOTOELECTROCHEMICAL PERFORMANCE OF NI-OXIDE-COATED N-SI PHOTOANODES BY ATOMIC-LAYER DEPOSITION OF ULTRATHIN FILMS OF COBALT OXIDE
Xinghao Zhou, Rui Liu, Ke Sun, Dennis Friedrich, Matthew T. McDowell, Fan Yang, Stefan T. Omelchenko, Fadl H. Saadi, Adam C. Nielander,Sisir Yalamanchili, Kimberly M. Papadantonakis, Bruce S. Brunschwigae and Nathan S. Lewis
Energy & Environmental Science (2015), 8, 2644.
DOI: 10.1039/c5ee01687h
(view full article: http://pubs.rsc.org/en/content/articlehtml/2015/ee/c5ee01687h)


Producing fuels from sunlight, water and carbon dioxide may provide a path to addressing the grand challenges of clean energy and climate change. Sunlight-driven water splitting is a key step in the fuel-forming reactions. Semiconductors such as Si hold promise as light absorbers but require protection from corrosion. While NiOx coatings work for protecting np+ photoanodes, their use on n-type absorbers suffers from the non-optimal interfacial energetics. In the present work, Zhou et al use Kelvin Probe Force Microscopy (KPFM) to investigate the energetics resulting upon introduction of a CoOx film between the Si and NiOx. The KPFM measurements show a 120mV increase in workfunction, in accordance with the negative shift in flat-band potential and the current density-voltage behavior observed in 1.0 M KOH, explaining the stable, high performance observed in this device structure.