pubs.acs.org/NanoLett
New Flexible Toolbox for Nanomechanical Measurements with Extreme Precision and at Very High Frequencies Alexander Fian,† Monica Lexholm,† Rainer Timm,† Bernhard Mandl,†,‡ Ulf Håkanson,† Dan Hessman,† Edvin Lundgren,† Lars Samuelson,† and Anders Mikkelsen*,† †
Department of Physics, Lund University, Box 118, 22100 Lund, Sweden, and ‡ Institute for Semiconductor and Solid State Physics, University Linz, Altenbergerstrasse 69, A-4040 Linz ABSTRACT We show that the principally two-dimensional (2D) scanning tunneling microscope (STM) can be used for imaging of 1D micrometer high free-standing nanowires. We can then determine nanowire megahertz resonance frequencies, image their top-view 2D resonance shapes, and investigate axial stress on the nanoscale. Importantly, we demonstrate the extreme sensitivity of electron tunneling even at very high frequencies by measuring resonances at hundreds of megahertz with a precision far below the angstrom scale. KEYWORDS Nanomechanics, angstrom precision, scanning tunneling microscopy, semiconductor nanowire, parametric resonance
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tude of quantum phenomena can be tailored in the nanowires.5 Studies of nanowires for mechanical experiments will allow us to understand the interplay between mechanical properties and complex structural features of the wires.
* To whom correspondence should be addressed. E-mail: anders.mikkelsen@ sljus.lu.se. Received for review: 04/30/2010 Published on Web: 08/26/2010
The common approach for studying mechanical properties on the nanoscale has been to lithographically design complex devices that can determine resonance frequencies with high precision using electrical, magnetic, or optical transduction.1-4,7,8 These skillfully crafted structures have been very successful in probing the nanomechanical regime, however due to the amount of engineering necessary to make them, testing a wide range of materials, structures, and experimental configurations will be difficult. Direct measurement with a versatile tool that can be applied to freestanding nanowires would be an important alternative. Mechanical motion studies on free-standing nanoscale objects have been done with time averaging detection by electron microscopy9-12 and optical methods such as stroboscopic imaging.13,14 But, to study objects with dramatically smaller vibration amplitudes in the quantum limit, these methods have significant shortcomings in terms of the needed ultrahigh resolution, extremely low temperatures or rigorous environment control.1,2 The exponential dependence of tunneling current with gap distance in STM makes the detection of extremely small changes in vibrational amplitudes possible, even in the gigahertz range.1 In addition, very low temperatures in the milliKelvin regime can be reached,15 and the environment can be tightly controlled. While some work has been done on 40 nm nanowires lying down on a substrate,16 no STM studies have been reported on upright standing nanowires. This is no wonder as the STM is a tool traditionally restricted to two-dimensional (2D) flat surfaces.17 However realizing 1D imaging and dynamical
ecent demonstrations and use of mechanics at the nanoscale, approaching the quantum mechanical limit, rely on elaborate design of special device structures.1-4 The reason is stringent demands of very high resonance frequencies and the extremely small movements of objects governed by quantum mechanics or used for a variety of future nanoelectromechanical devices1,2 Studying quantum phenomena related to mechanical resonator systems is a daunting task as they can usually only be observed for kT < hω. Thus, even at frequencies ω around 1 Gigahertz, temperatures T below hundred milliKelvin are needed and the resulting relevant motion is typically far below the angstrom level.2 To go beyond single demonstration experiments, methods allowing easy exchange and large variability in the studied mechanical systems, while retaining extreme sensitivity, would be necessary. One type of nanostructure well suited for such studies, is crystalline free-standing nanowires grown by self-assembly techniques with precise control down to the atomic scale.5 Growing nanowires with relevant resonance frequencies is no problem today. The fundamental resonance frequency of a 100 nm long silicon nanowire with a diameter of 10 nm will be in the gigahertz range.6 State-of-the-art semiconductor nanowires can be grown virtually defect free with diameters from hundreds down to a few nanometers with lengths up to many micrometers.5 A wide range of materials can be used and complex heterostructures with atomic scale precision can be grown, allowing an extreme variability of mechanical properties. In addition, complex structures displaying a multi-
© 2010 American Chemical Society
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DOI: 10.1021/nl1015427 | Nano Lett. 2010, 10, 3893–3898
mechanical probing with the STM would be an important step toward flexible studies with very high precision, potentially of structures governed by quantum mechanical phenomena. In the present work, we show that it is possible to use a commercial STM with little modification to image micrometer high free-standing InAs nanowires. It can be noted that the method is of course not particularly limited to InAs nanowires, any type of reasonably conducting nanowire can be used. Further we show how very high resonance frequencies can be measured and how changes in resonance amplitudes can be measured with extreme precision in a time averaged fashion. Finally we discuss the relevance of the present setup to measurements in the quantum mechanical regime and possible technical improvements. InAs nanowires have been epitaxially grown from Aerosol gold particles using chemical beam epitaxy.18 The diameter of the nanowire is dictated by the size of the gold particle and the length can be controlled through the growth time. To simplify the STM measurements, gold particles are only deposited on half of the sample. The nanowires have been grown at 440 °C for 20 min with a chamber pressure
