Laboratory for Surface Science – Scanning Probe Microscopy, Molecular beam Epitaxy and Plasmonics in 2D Materials
Scientific head: Ch. Deneke
User contact: C. Costa
Rua Giuseppe Máximo Scolfaro 10000
Campinas – SP – CEP 13083-100, Brazil
Tel.: +55 19 3512 1166
The Laboratory for Surface Science (Laboratório de Ciência de Superfícies – LCS) is an open user facility run by the CNPEM as part of the LNNano. We offer a variety of scanning probe microscopy techniques for users. We carrying out research in the areas of SPM, semiconductor nanostructures grown by molecular beam epitaxy (MBE) as well as plasmonic and optical properties of 2D materials. Proposals to use the instruments can be submitted over the user portal.
First publication of the MBE: Fabrication and Optical Properties of Strainfree Self-assembled Mesoscopic GaAs Structures
We use a combined process of Ga-assisted deoxidation and local droplet etching to fabricate unstrained mesoscopic GaAs/AlGaAs structures exhibiting a high shape anisotropy with a length up to 1.2 μm and a width of 150 nm. We demonstrate good controllability over size and morphology of the mesoscopic structures by tuning the growth parameters. Our growth method yields structures, which are coupled to a surrounding quantum well and present unique optical emission features. Microscopic and optical analysis of single structures allows us to demonstrate that single structure emission originates from two different confinement regions, which are spectrally separated and show sharp excitonic lines. Photoluminescence is detected up to room temperature making the structures the ideal candidates for strain-free light emitting/detecting devices
Nanoscale Research Letters (2017) 12:61; DOI: DOI 10.1186/s11671-016-1782-1
Results from our collaborations: Antimonide-based membranes synthesis integration and strain engineering
In this work, we present a versatile method to fabricate antimonide-based heterostructures in membrane form, and we demonstrate the potential of these materials to enable hybrid integration and elastic strain engineering. Integration of Sb-based compound membranes with Si substrates will potentially solve a number of technological challenges in the fabrication of IR optoelectronic devices based on type II superlattices. Furthermore, transfer of Sb compounds to insulating materials will enable a thorough investigation of electrical transport in the heterostructure via Hall and Van der Pauw measurements. Finally, membrane technology applied to Sb-based structures will enable one to engineer strain distributions, which are not obtainable within the limitations of epitaxial growth processes.
(A) Coplanar diffraction pattern acquired around the (004) GaSb peak of the as-grown InAs/GaSb T2SL (solid black circles). A fit to the experimental data is plotted as a red line. (Inset) The pattern obtained by grazing-incidence (GI)-XRD around the (220) reflection of the as-grown (circle-line pattern) and the transferred sample (triangle-line pattern). (B) Coplanar XRD pattern acquired from a transferred InAs/GaSb SL on a Si substrate (solid black circles).
XRD data obtained at XRD2 of the LNLS and analyzed by Ch. Deneke (LNNano) and S. O. Ferreira (UFV). Sample preparation, design of experiments and initiation of the collaboration by our friends from the University of New Mexico, Albuquerque, USA.
Published in: Marziyeh Zamiri, Farhana Anwar, Brianna A. Klein, Amin Rasoulof, Noel M. Dawson, Ted Schuler-Sandy, Christoph F. Deneke, Sukarno O. Ferreira, Francesca Cavallo, and Sanjay Krishna PNAS 2016 ; published ahead of print December 16, 2016, doi:10.1073/pnas.1615645114
F.C. and S.K. designed research; M.Z., F.A., A.R., N.M.D., T.S.-S., and S.O.F. performed research; M.Z. and A.R. performed continuum mechanical modeling; B.A.K. and T.S.-S. performed material growth; N.M.D. performed atomic force microscopy; C.F.D. and S.O.F. performed X-ray diffraction measurements and data analysis; M.Z. and F.C. analyzed data; and M.Z., B.A.K., C.F.D., F.C., and S.K. wrote the paper.
Seeing the Surface-Plasmon-Phonon (SPP) coupling of graphene to SiO2
Using the Scanning Nearfield Optical Microscope part of the new Anasys NanoIR2-s instrument, we imaged the SPP coupling in a graphene flake provided by the UFMG using a CO2 laser. The image shows that we can directly access the optical properties of this material using the technique and obtain a deeper inside into its properties.
We see in the middle of the topography image a single layer graphene flak. The middle image and right image depict the optical near field image with a resolution of ca. 20 nm of the graphene flake. We can observe in the optical images a standing wave structure (changing bright contrast from the middle to a double structure on the sample boarders) in dependence of the graphene flake width, which arises from resonances of the graphene surface plasmon excited by a the indecent light and the plasmon wave being confined by the geometry of the graphene flake. Hence just standing plasmon waves can form on the surface with a wavelength fitting into the graphene flake (like a sound wave inside a organ pipe).
SNOM images acquired by Evandro M. Lanzoni, Carlos A. Costa, and Christoph Deneke using the Anasys NanoIR2-s. Sample prepared by Alisson Cadore and Ingrid Barcelos (Federal University of Minas Gerais).
Local infra-red spectroscopy using the thermal response
To be complementary to the existing scanning probe microscopy techniques of the LNNano and the LNLS, the LCS of the LNNano installed a NanoIR2-s instrument from Anasys using SISNano resources. The instrument was delivered and setup in Oct. 2016.
The instrument uses the local thermal response of a material to excite a resonance vibration in a contact mode AFM cantilever. The amplitude of the excited vibration is proportional to the local infra-red absorption. Tuning the wavelength of the used light source (a quantum cascade laser operating between 1450 to 1860 cm-1) allows to obtain a local infra-red spectrum identical to the infra-red fingerprint known from far field absorption spectroscopy with a resolution of ca. 100 nm. Hence, the approach is complimentary to the scanning near field microscopy (SNOM) overcoming the classical diffraction limit of fare field spectroscopy. Whereas SNOM – as implemented at the IR beamline of the LNLS – is most efficient for inorganic samples and plasmonic application, this technique allows easy and solid access to soft matter and biological samples enabling IR based fingerprinting for this material class.
The instrument will have a SNOM detection side working with a single frequency laser permitting pre-characterization of samples before entering the IR beamline SNOM for detailed analysis.
One of the first test using a PMMA, Polystyrene in Epoxy standard sample. We can obtain IR spectra from this material, which exhibit either a peak at 1728 cm-1, 1605 cm-1 or 1600 cm-1 specific for the material and their position. Imaging using this wavelength, we obtain chemical composition maps as demonstrated above. For use, samples have to be prepared carefully.
AFMIR images acquired by Evandro M. Lanzoni, Carlos A. Costa, and Christoph Deneke using the Anasys NanoIR2-s.
User meeting 2015 – AFM Workshop 2015
We had a nice AFM Workshop 2015 from the 14. December to the 16. December. More than 70 persons showed up and joined the talks, practical courses and social events. We will put some photos during the next day onto the Workshop webpage, but below is already the group photo taken yesterday. Thanks everyone for coming and thanks for all helping making the event possible.
The birth of our new molecular beam epitaxy facility
See the setup and drive in of our new 2″ III-V molecular beam epitaxy machine. We hope that the machine will provide high quality samples for our internal research as well as the Brazilian semiconductor community.
Thanks to all the people involve making this happen.
Results by our Users: Determination of High-Frequency Dielectric Constant and Surface Potential of Graphene Oxide and Influence of Humidity by Kelvin Probe Force Microscopy
We use Kelvin probe force microscopy (KPFM) and capacitance coupling (dC/dz) to study the electrical properties of graphene oxide (GO). We propose using the dC/dz signal to probe the high frequency dielectric constant of mono- and few-layer GO. Our measurements suggest that the dynamic dielectric constant of GO is on the order of εGO ≅ 3.0 ε0, in the high frequency limit, and independent of the number of GO layers. The measurements are performed at a humidity controlled environment (5% of humidity). The effects of increasing humidity on both the dC/dz and KPFM measurements are analyzed.
Langmuir, Article ASAP, DOI: 10.1021/acs.langmuir.5b01786
AFM images acquired by Francisco C. Salomão, Evandro M. Lanzoni, Carlos A. Costa, Christoph Deneke, and Eduardo B. Barros using the Park NX10.
Results by our Users: Enhanced photovoltaic performance of inverted hybrid bulk-heterojunction solar cells using TiO2/reduced graphene oxide films as electron transport layers
In this study, we investigated inverted hybrid bulk-heterojunction solar cells with the following configuration: fluorine-doped tin oxide (FTO) jTiO2∕RGOjP3HT∶PC61BMjV2O5 or PEDOT:PSS|Ag. The TiO2∕GO dispersions were prepared by sol-gel method, employing titanium isopropoxide and graphene oxide (GO) as starting materials. The GO concentration was varied from 0.1 to 4.0 wt%. The corresponding dispersions were spin-coated onto FTO substrates and a thermal treatment was performed to remove organic materials and to reduce GO to reduced graphene oxide (RGO). The TiO2∕RGO films were characterized by x-ray diffraction, Raman spectroscopy, and microscopy techniques. Atomic force microscopy (AFM) images showed that the addition of RGO significantly changes the morphology of the TiO2 films, with loss of uniformity and increase in surface roughness. Independent of the use of V2O5 or PEDOT: PSS films as the hole transport layer, the incorporation of 2.0 wt% of RGO into TiO2 films was the optimal concentration for the best organic photovoltaic performance. The solar cells based on TiO 2∕RGO (2.0 wt%) electrode exhibited a ∼22.3% and ∼28.9% short circuit current density (Jsc) and a power conversion efficiency enhancement, respectively, if compared with the devices based on pure TiO2 films. Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO2 films can promote the appearance of regions with different charge dissipation capacities.
Journal of Photonics for Energy, 5, 057408 (2015); DOI:10.1117/1.JPE.5.057408
AFM images acquired by Andreia Morais, João Paulo C. Alves, Francisco Anderson S. Lima, Monica Lira-Cantu, and Ana Flavia Nogueira using the Park NX10.
Graphene/h-BN Plasmon-phonon coupling and plasmon delocalization observed by infrared nano-spectroscopy
We observed the coupling of exfoliated graphene Dirac plasmons to different surfaces using scattering-type scanning near-field optical microscopy integrated into a mid-infrared synchrotron-based beamline. A systematic investigation of a graphene/hexagonal boron nitride (h-BN) heterostructure is carried out and compared to the well-known graphene/SiO2 heterostructure. Broadband infrared scanning near-field optical microscopy imaging is able to distinguish between the graphene/h-BN and the graphene/SiO2 heterostructure as well as differentiate between graphene stacks with different numbers of layers. Based on synchrotron infrared nanospectroscopy experiments, we observe a coupling of surface plasmons of graphene and phonon polaritons of h-BN (SPPP). An enhancement of the optical band at 817 cm-1 is observed at graphene/h-BN heterostructures in consequence of hybridization between graphene plasmons and longitudinal optical phonons of h-BN. Furthermore, longitudinal optical h-BN modes are preserved on suspended graphene regions (bubbles) where the graphene sheet is tens of nanometers away from the surface while the amplitude of transverse optical h-BN modes decrease.
Nanoscale Accepted Manuscript (2015); DOI:10.1039/C5NR01056J
Images acquired by Ingrid Barcelos and Christoph Deneke using the LNLS IR beamline (Neaspec Instrument).
Results by our Users: Self-Assembly of a Designed Alternating Arginine/Phenylalanine Oligopeptide
A model octapeptide peptide consisting of an alternating sequence of arginine (Arg) and phenylalanine (Phe) residues, namely, [Arg-Phe]4, was prepared, and its self-assembly in solution studied. The simple alternating [Arg-Phe]4 peptide sequence allows for unique insights into the aggregation process and the structure of the self-assembled motifs. Fluorescence and UV–vis assays were used to determine critical aggregation concentrations, corresponding to the formation of oligomeric species and β-sheet rich structures organized into both spheroidal aggregates and highly ordered fibrils. Electron and atomic force microscopy images show globular aggregates and long unbranched fibers with diameters ranging from ∼4 nm up to ∼40 nm. Infrared and circular dichroism spectroscopy show the formation of β-sheet structures. X-ray diffraction on oriented stalks show that the peptide fibers have an internal lamellar structure, with an orthorhombic unit cell with parameters a ∼ 27.6 Å, b ∼ 9.7 Å, and c ∼ 9.6 Å. In situ small-angle X-ray scattering (SAXS) shows the presence of low molecular weight oligomers in equilibrium with mature fibers which are likely made up from 5 or 6 intertwined protofilaments. Finally, weak gel solutions are probed under gentle shear, suggesting the ability of these arginine-rich fibers to form networks.
Langmuir, Article ASAP (2015); DOI:10.1021/acs.langmuir.5b00253
AFM images acquired by Carla C. Decandio, Emerson R. Silva, Ian W. Hamley, Valeria Castelletto, Michelle S. Liberato, Vani X. Oliveira, Jr., Cristiano L. P. Oliveira, and Wendel A. Alves using the DI Multimode Nanoscope IIIa.
Talk at the WIN, Canada
Ch. Deneke had the opportunity to talk about his research, the institute and the group in Canada at the Waterloo Institute of Nanotechnology. Above, we provide the link to the talk, which is online at YouTube. Thanks again for the invention and the warm welcome there.
Images (c) by WIN, Feb. 2015. Christoph Deneke (left) and Arthur Carty (right; Director of the WIN).
AFM course during the MicroMAT 2014
From the 19. to the 20. November a introduction course to various AFM based techniques took place at the labs of the LCS. Participant had the chance, to learn about basic atomic force microscopy as well as more advanced applications like KFM, MFM, EFM and working in liquid. We hope, all participants enjoyed the course.
The top image shows a lithographic pattern written by switching the ferro-electric domains of a PZT test sample during the course. The surface was than imaged using DC-EFM and the domain structure visualized. Beside, we have the group image of the participants of the course (click to enlarge).
Images: (c) by LCS group, November 2014.
DC-EFM image and lithography by E. Lanzoni and Ch. Deneke using the Park NX10.
InAs migration on released, wrinkled InGaAs membranes used as virtual substrate
Partly released, relaxed and wrinkled InGaAs membranes are used as virtual substrates for overgrowth with InAs. Such samples exhibit different lattice parameters for the unreleased epitaxial parts, the released flat, back-bond areas and the released wrinkled areas. A large InAs migration towards the released membrane is observed with a material accumulation on top of the freestanding wrinkles during overgrowth. A semi-quantitative analysis of the misfit strain shows that the material migrates to the areas of the sample with the lowest misfit strain, which we consider as the areas of the lowest chemical potential of the surface. Material migration is also observed for the edge-supported, freestanding InGaAs membranes found on these samples. Our results show that the released, wrinkled nanomembranes offer a growth template for InAs deposition that fundamentally changes the migration behavior of the deposited material on the growth surface.
Nanotechnology 25, 455603 (2014); DOI:10.1088/0957-4484/25/45/455603
Images acquired by S. Filipe Covre da Silva and E. M. Lanzoni using the Park NX10.
Seeing atoms using the atomic force microscope
Pushing our instruments to their resolution limit, we demonstrated atomic resolution using atomic force microscopy. Using the error signal in a fast scan mode, we are able to see the atomic surface lattice of a HOPG sample. This exercise shows that the environment and instrumentation of the group is good enough to obtain maximal possible resolution.
Images: (c) by LCS group, October 2014.
Images acquired by Evandro Martin Lanzoni and Carlos Costa using the DI Multimode Nanoscope IIIa.
Software Contribution: Module in Gwyddion
We finally managed to contribute code to the Gywiddion code base. Grain marking using Otsu’s method was implemented by our group member Vinicius and is now available in Gywiddion 2.34 (see release notes). We hope to further contribute to this open source software freely available to the SPM community making it better for everybody (and our specific problems).
Gwyddion release 2.34 is available on the 15. Dec. 2013
Mismatched Heteroepitaxial Growth: InAs Islands on Compliant Si Substrates
Freestanding, edge-supported silicon nanomembranes are defined by selective underetching of patterned silicon-on-insulator substrates. The membranes are afterward introduced into a molecular beam epitaxy chamber and overgrown with InAs, resulting in the formation of InAs islands on flat areas and at the top of the Si nanomembranes. A detailed analysis of sample morphology, island structure, and strain is carried out. Scanning electron microscopy shows that the membrane stays intact during overgrowth. Atomic force microscopy reveals a lower island density on top of the freestanding membranes, denoting a modified wetting or diffusivity in these areas. An observed bending of the membrane indicates a strain transfer from the InAs islands to the compliant substrate. X-ray diffraction and finite-element modeling indicate a nonuniform strain state of the island ensemble grown on the freestanding membrane. A simulation of the bending of the nanomembranes indicates that the islands at the center of the reestanding area are highly strained, whereas islands on the border tend to be fully relaxed. Finally, continuum elasticity calculations suggest that for a sufficiently thin membrane InAs could transfer enough strain to the membrane to allow coherent epitaxial growth, something not possible on bulk substrates.
ACS Nano 5, 10287 (2012); DOI: 10.1021/nn304151j
Image acquired using our DI instrument.