The Grutter group
tries to push the limits of instrumentation and is one of the internationally leading groups in the development of atomic force microscope (AFM) techniques and its application to understanding how nanoscale objects can be used for information storage and processing (the field commonly known as nanoelectronics). We build and operate instruments at the absolute limits given by nature - this challenges creativity, physical insight and technological wizardry. AFMs are a unique tool for the nanoscale: they are capable of imaging, measuring properties and manipulating nano objects such as single electrons, individual molecules or single synapses in almost any environment. As a result, one can discover how atomic scale structure relates to exciting emerging nanoscale properties of matter. A dynamic, diverse, creative and highly collaborative team of students builds or adapts AFM hardware to investigate:
Quantum dots (QDs) are often referred to as ‘artificial atoms’ as they exhibit the discrete energy levels in their electronic structure as atoms do due to their small size. It is important to understand how the structure of QDs such as shape and size is related to their electronic structure as the variation of the energy levels is technically relevant in such applications as semiconductor lasers and quantum cellular automata. Electrostatic force microscopy (EFM) is a powerful tool to investigate the electrical properties of nanometer-scale electronic devices.
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We have experimentally developed EFM instrumentation to allow us thus detected the energy levels of single electrons in a QD, a molecule or a single dopant by measuring the electrostatic force from the electric charge using a cryogenic atomic force microscope (AFM). Recently, we have also achieved single electron spectroscopy capabilities at room temperature using an ultra high vacuum AFM coupled to a surface science system. Theoretical understanding and quantitative modeling is done in close collaboration with the theory groups of Prof. A. Clerk., Hong Guo, Alex Schluger and Kirk Bevan.
Our AFM oscillates a cantilever with a sharp metallic tip at distances on the order of nanometers above the sample. We monitor the shift in resonance frequency of the cantilever, which is caused by the change in the tip-sample interaction force. In particular, the electrostatic force is dominant under the application of the tip-sample bias voltage. By scanning the tip over the sample, one can build a spatial map of the electrostatic force on the sample with nanometer resolution, or the tip can be held in one place to gain information on one location.
By positioning the tip over a InAs QD and sweeping the bias voltage, discrete shifts in the cantilever frequency occur which indicate sudden changes in the force. These changes are the result of a single electron entering (or leaving) the QD. The simultaneously measured damping of the cantilever shows peaks at the same voltages where the electrons entered the QD. This means that a part of the energy in the oscillating cantilever is transferred to the charge in the QD. Fundamentally we are measuring the back-action of the quantum system on the macroscopic oscillator (the AFM cantilever). In these measurements, the AFM tip is acting both as a scannable gate and a charge detector with single electron sensitivity.
The information that can be extracted from the peak position and shape, which vary with external control parameters (such as oscillation amplitude (which controls the QD – cantilever coupling strength) or temperature) includes the Coulomb blockade energy of QDs and their energy level spacing, coupling strengths between dots and other interesting parameters. Furthermore, we can directly determine the energy level and its degeneracy (e.g. one can distinguish the 1s from the 2s electron)! This recent demonstration opens the possibility of performing electron spectroscopy with atomic resolution.
Most recently, we have used this technique to perform a redox reaction at the single molecule level. This allows us to extract important site dependant quantities such as the reorganization energy, electron-phonon coupling or even the energies of molecular vibrations!
We are currently working determining if we can use this technique to understand charge transfer in organic photovoltaic systems (the light gathering protein antenna complex). We also plan to investigate coupled InAs quantum dots at 4.5K to experimentally verify if we can use this technique to determine electron coherence times. This would be a very exciting demonstration of a theoretical prediction by one of our collaborators (Prof. Aashish Clerk). We are interested in experimentally using this technique to investigate charge transfer between organic molecules and electrodes (relevant for molecular electronics and the efficiency of organic photo voltaic systems) or gold nanoparticles on MgO surfaces (relevant for understanding the fundamental properties of this fascinating catalytic system). Finally, in collaboration with Prof. Neil Curson ad Taylor Stock (University College London, Nanocenter) we are exploring the possibility of using AFM to perform spectroscopy on single P or As dopant atoms in silicon.
See the recent NSERC News flash
In AFM, it is well known that imaging rates are mechanically limited by the resonance frequency of the cantilever (typically 0.1-1 MHz). This limitation, however, does not apply to measuring properties. Time resolution is only limited by the accuracy of the measured average AFM signal after a step-response or repetitive pump-probe excitation of a sample property!
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We have developed various electrical excitation techniques to measure ion mobility in oxides or Li battery cathodes. We have combined our AFM with a 100fs laser optical pump-probe excitation system. This allows us to measure the dynamics of evolving nonlinear polarizations arising from resonant and non-resonant particle excitations in materials. This exciting development will allow us to correlate atomic scale structure with properties measured on ultra-fast time scale limited by the fastest sample excitation available. Motion of single molecules, conformation changes during chemical reactions, electron dynamics in solids, and the effects of defects or trap states on electron motion and behavior, are amongst many fundamental processes waiting to be observed at the femtosecond and nanometer scale.
By measuring properties on ultrafast time scales with nanometer spatial resolution, we will address fundamental science questions on the role defects play in organic photovoltaic materials, including: How does structure (including defects) and environment determine charge separation and recombination kinetics? What are the relevant charge transport mechanisms, and what structures dominate transport rates? How important are coherence and vibronic phenomena? What is the role of static and dynamic disorder?
We are also planning on using our ultrafast AFM methods to study the mobility and spatially resolved activation barriers of vacancies in oxide materials such as TiO2, SrTiO3 and other perovskites (collaboration with Profs. Kirk Bevan (McGill Materials Eng.), Alex Shluger (University College London) and Geoff Thornton (University College London). We will also continue to develop a better quantitative understanding of how the ultrafast non-linear sample interactions measured by AFM can be interpreted quantitatively by collaborating with ultrafast optical experiments groups (Prof. David Cooke, Physics McGill) and various theory groups.
Most recently, we have started to investigate the ultrafast optoelectronic properties of defects in 2D systems. Charge separation, exciton diffusion and charge recombination can be observed in real space and real-time by using our 10fs laser as a pump and the force detected non-linear optical interactions in the sample as a probe. This project is being undertaken in collaboration with Alex Schluger (theory, University College London) and expert material growers Kuan Eng Johnson Goh and Wong Pei Yu Calvin in Singapore. Furthermore, we are collaborating with Adina Luican-Mayer (U. Ottowa) to generate defect arrays on these 2D materials using scanning probe lithography.
Recent Publications:
Y. Miyahara and P. Grutter
Chapter in Kelvin probe force microscopy - from single charge detection to device characterization; Editors S. Sadewasser and T. Glatzel, Springer-Verlag (2018)
A. Mascaro, Z. Wang, P. Hovington, Y. Miyahara, A. Paolella, V. Gariépy, Z. Feng, T. Enright, C. Aiken, K. Zaghib, K.H.Bevan, and P. Grutter
Nano Lett. 17, 4489 (2017)
A very common interface is the solid-liquid interface. Most surface science tools, which rely on electron probes, do not function in this environment crucial to many fundamental as well as applied questions in engineering and biology. We have built and developed advanced AFM methods enabling us to image, characterize and manipulate in solution under full electrochemical control. This allows the simultaneous observation of the structure of the liquid electrolyte near a solid electrode with atomic scale lateral resolution as well as the associated electrode surface structure and mechanical properties at the same length scale.
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We have validated this method by observing atomic scale friction on surfaces modified electrochemically (collaboration with Prof. R. Bennewitz, Leibniz-Institut für Neue Materialien GmbH, Saarbruecken (D)) and by determining the structure of water layers above a surface with atomic lateral resolution (in collaboration with Prof. H. Yamada, Kyoto University). We also collaborate with theorist Prof K. Bevan (Materials Eng., McGill) and Dr. K. Zaghib (Hydro Quebec Research Inst.) to investigate Li diffusion to understand quantitatively some of the basic steps in Li batteries. Note that Hydro Quebec is one of the leading centers worldwide in the field of Li batteries, spanning fundamental materials research to systems integration and testing, including a large patent portfolio. Our time resolved ion conductance AFM studies are aimed at providing guidance and validation of atomistic materials modeling. This will provide a better understanding of nanomaterials phenomena relevant to the design of fast charging and high energy density cathodes. In this regard, scanning probe microscopy is an ideal tool for Li-ion cathode mapping, as it allows one to perform imaging, manipulation and measurements from the mesoscopic submicron regime down to the atomic level.
Long-range electron transfer is a ubiquitous process that plays an important role in electrochemistry, biochemistry, organic electronics, and single molecule electronics. Fundamentally, quantum mechanical processes, at their core, manifest through both electron tunneling and the
associated transition between quantized nuclear vibronic states (intramolecular vibrational relaxation) mediated by electron−nuclear coupling. We have measured the long range
electron transfer at the interface between a single ferrocene molecule and a gold substrate separated by a hexadecanethiol quantum tunneling barrier using AFM as a gate and charge sensor.
These experiments link electrochemistry and single molecular electron transport.
The most recent project is a collaboration between Machine Learning experts Julio Valdes and Alain Tchagang (NRC) and Kirk Bevan (Materials Engineering, McGill). We plan to advance CO2-recycling via electrocatalysis through AI driven
mapping and engineering of its key reaction centres at the atomic scale. Copper is universally
agreed to be by far the most effective CO2-to-fuel electrocatalyst. However, no systematic
approach has yet been developed to engineer its governing reaction mechanisms. To address
this gap, we shall utilize AI to accelerate the design of chemically active sites (and interfacial
solvation structures) on copper during CO2 conversion. This will be accomplished through a
combination of AI methods to guide and accelerate the operando atomic scale imaging and
spectral characterization of CO2-to-fuel electrocatalysis atop copper. Using supporting ab-initio
modeling we will further tailor surface and solvation enhancements in CO2-to-fuel
electrocatalysis based on experimentally determined structures and chemical composition. In
concert with big-data and machine learning methods, we shall shall utilize AI to establish
predictive design correlations between experimental and theoretical descriptors. Through
these three research pillars we aim to rapidly engineer the physicochemical operation of CO2-
to-fuel electrocatalysis, thereby yielding new AI driven routes towards efficiency and stability
engineering.
Atomic Force Microscopy, in combination with single molecular fluorescence techniques, is a powerful tool to study multiple problems in biology. Many fundamental biological questions can be investigated from a new perspective by determining interaction forces on the molecular scale or the cellular level by AFM and correlating them with various more standard optical techniques (such as multi-wavelength fluorescence, TIRF and single photon counting techniques), biochemical methods, electrophysiology and nano/microfabrication. Our BioAFMs have been used to image biomolecular processes at the molecular level (such as compactification of DNA), determine viscoeleastic properties of smooth muscle cells relevant for asthma, mechanical property changes during synapse formation or manipulation of neuritis as well as photophysical properties of semiconductor quantum dots. This facility is also operated as a user facility, where we provide the instrumentation expertise and projects are defined and performed in collaboration with researchers in the life sciences.
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The AFM Facility for the Life Sciences is optimized for AFM applications in the life sciences by combining AFM with single photon fluorescence optical methods and electrophysiological measurements using patch clamp techniques. The facility is compatible with operation in solution under temperature control with exchange of drugs (home built temperature controlled profusion cell), allowing experiments to be performed in-vitro on live, cultured cells.
We have observed the changes in mechanical properties upon synapse formation in cultured rat neurons, determined the critical pressure and the origin of the failure mechanism leading to axonal death, developed a method to investigate the synapse recruitment time of proteins implicated in synapse formation and finally have demonstrated that AFM and related methods can be used to manipulating and reconnect live neurons! This might allow us to mechanically construct neuronal circuits. These experiments are characterized by close interactions with some of the leading researchers in the life sciences and chemistry, AFM methods development and fabrication and application of microfluidics systems.
M.H. Magdesian, M. Lopez, M. Mori, D. Boudreau, D. Oliver, A. Goulet-Hanssens, D. Gobert, W. Paul, X.Y. Xua, R. Sanz, Y. Miyahara, J.-F. Desjardins, C.J. Barrett, E. Ruthazer, A. Fournier, Y. De Koninck, P. Grutter
J. Neuroscience 36,979 (2016)
F. Suarez, P. Thostrup, D. Colman , P. Grutter
Developmental Neurobiology, 73, 98-106 (2013)
Electro-mechanical contacts at the atomic scale
This project focuses on three main subjects – Contact formation at the atomic level, Nano indentation and the deeper understanding of Scanning Tunneling Microscope/Atomic Force Microscope (STM/AFM) contrast mechanisms. In order to study atomically characterized systems, we use a homebuilt instrument combining a Scanning Tunneling Microscope (STM), an Atomic Force Microscopy (AFM) and a Field Ion Microscope (FIM). This allows us to study the mechanical and electronic interactions of atomically defined systems.
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We have developed techniques to manipulate and characterize the atomic structure of tips by using FIM. Figure 1 shows a sequence of FIM images. The bright spots in the center of the image are individual tungsten atoms. We can also modify the tip structure in a controlled way with this technique by applying suitable voltage pulses. We can thus machine a tip atom by atom! Using such a tip one can also characterize the sample surface on an atomic scale using STM or AFM.
The main focus of our current research is to understand the mechanical and electronic properties of atomic scale contacts to surfaces. This is a necessary step to understanding the effect of contacts on molecules – the key issue in molecular electronics. Molecular electronics is a promising new field of Physics where quantum transport phenomena in individual molecules can be observed, measured and used. Understanding quantum transport effects in nanostructures and molecules is both invaluable to fundamental science and of critical importance to modern information technology. The ongoing trend of electronic device miniaturization will soon lead to devices with transport characteristics that are dominated by quantum effects. From a fundamental perspective there is a severe lack of experimental techniques that can make measurements of electron transport in molecular systems with accurate knowledge of the atomic details. Currently available experimental techniques, such as break junctions, provide little insight into crucial details of the system such as bonding positions and molecular orientation – even the number of atoms in a break junction is not controlled. Theoretical calculations have demonstrated that transport characteristics are often dominated by details of the contacts. Our experiments allow a critical, no fit parameter test of theoretical models.
By comparing to molecular dynamics simulations (in collaboration with Y. Qi, GM Research Labs) and ab-initio transport modelling (collaboration with Prof. H. Guo, Physics McGill) these measurements allow us to understand in detail how the contact forms, in particular the subtle interplay between atomic scale structure and transport. A fascinating question, to be answered soon, is what changes if a molecule is sandwiched between this atomically defined, all-metal junction.
Investigating the onset of indentation and the formation of dislocations in metals is traditionally done by Nano indentation. Our three atom tip is the smallest and best defined nano indenter possible. By pressing it in a controlled fashion into an atomically flat sample many questions in that field can be addressed. Will the behaviour at the atomic scale be similar to the macroscopic experiments Are there phenomena not seen in modelling due to size or time constraints?
Our last focus of interest is the deeper understanding of STM contrast mechanisms, in particular we plan on investigating how the tip atomic structure influences key I-V spectroscopy measurements. While this technique is very powerful and widely used, some aspects of the measurement are still not fully understood, in particular the role of force.
D. J. Oliver, J. Maassen, M. El Ouali, W. Paul, T. Hagedorn, Y. Miyahara, Y. Qi, H. Guo, and P. Grütter
Proc. Natl. Acad. Sci. U. S. A. 109, 19097 (2012); doi:10.1073/pnas.1208699109
We want to understand how the structure of molecules on a surface relate to properties. The advancement of molecular electronics depends upon an improved understanding of electronic transport mechanisms through single molecules. In order to compare to existing theoretical models, atomically well defined devices are needed as the effect of local structure is known to be significant. Similarly, the details of the individual steps leading to charge separation in organic molecule acceptor-donor structures interacting with light is poorly understood. The fundamental limitations of organic photovoltaic devices is thus not understood – but needs to be determined. We are laying the foundations to construct atomically defined molecular devices in a planar geometry on an insulator. One thus needs to operate in ultra-high vacuum (UHV) (to have an atomically clean environment), image the structures with atomic resolution (using high resolution noncontact AFM) and develop methods to electrical contact leads in UHV (a HUGE challenge).
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In the field of molecular electronics, where single molecules are utilized as the active components of an opto-electronic device, it is commonly acknowledged that the electrodes and environment play an important role in the properties of the device as well as the molecule. Due to this sensitivity to the local structure one must know the atomic scale structure of the device region, including the electrodes and substrate, in order to compare reliably with theoretical predictions of the transport properties. It is the central goal of this project to construct and characterize a single molecule device in order to advance understanding of molecular transport mechanisms. Understanding how atomic scale defects affect these properties (such as photovoltaic conversion efficiency) paves the path to developing devices better suited for applications.
The approach we take is to build a planar device, such that it can be characterized by scanning probe microscopy techniques and in the ultra-clean environment of UHV to eliminate unknown contaminants. We will also use an insulating surface, so that electronic transport can be measured without current bypass through the substrate. In order to construct such a device, a basic knowledge of growth processes and interactions of both molecules and metals on insulators must first be understood.
Using the non-contact atomic force microscope (nc-AFM), we have made considerable progress on understanding growth of molecules and metals on ionic surfaces. With this technique we are able to determine the molecular structure of organic deposits on alkali halides and elucidate important processes in growth. In order to control the growth, we have explored templating of the surface by creation of monatomic depth nanopits which trap molecules, as well as pre-seeding with metal deposits to initiate nucleation. Through a combination of these techniques we have proposed a strategy for creating a device structure on the nanoscale with well defined metal clusters acting as the final contact.
To create contacts from the mm scale down to the nanoscale in UHV, metal deposition through a shadow mask system is under development. Metal growth studies have allowed us to determine the best selections of metals to use for electrode patterning. Contacts to the sample holder are made in multiple mask steps with fine scale features created by deposition through a silicon membrane mask, fabricated in the Nanotools facility at McGill and with nanoscale features created by focused ion beam (FIB).
Having developed the basic surface science understanding required to construct a planar molecular device on an insulator, we will continue to seek methods of templating molecular growth by tuning interactions, and to refine the fabrication of in situ metal electrodes to measure conductivity. We are planning to develop time resolved Kelvin Probe Force Microscopy (a variant of Electrostatic Force Microscopy) to correlate structure with opto-electronic properties.
how single molecules conduct electricity, atomic structure dependence of the contacts, and interaction with light (relevant for understanding of charge transport in molecules or the fundamental limits of organic photovoltaics).
