Manipulation of nanoparticles using dynamic force microscopy: simulation and experiments

Dynamic force microscopy (DFM) in combination with special-purpose probe control software is used as a manipulation tool for the precise positioning of single gold nanoparticles on a mica substrate covered with a poly-Llysine film. Experimental results are presented that show how to construct arbitrary patterns of nanoparticles. The dynamic state of the cantilever during the manipulation process is studied experimentally by analyzing the simultaneously recorded non-contact amplitude and cantilever deflection. Numerical simulations guide and supplement the experiments in order to provide a physical description of the manipulation mechanism. The results presented here show that the nanoparticles are pushed along the surface once a critical contact force between tip and gold cluster is exceeded. In addition, a method for estimating the average separation between the tip apex and the sample in DFM is described. PACS: 07.79.Lh; 85.40.Ux; 07.05.Tp The study of nanoparticles and small molecular clusters is an area of current interest because of the unique properties of these materials [1, 2]. Colloidal nanoparticles are especially interesting because they can be produced in various well-controlled sizes and from various materials such as metals or semiconductors [3–5]. Nanoparticle patterns have a variety of potential applications, from data storage and single-electron transistors [6, 8, 9], to nanoelectromechanical systems (NEMS) fabrication, where they may serve as templates for growth or as nanocomponents that are assembled by robotic techniques to construct complex nanostructures from the bottom up. Building arbitrary patterns of nanoparticles requires precise positioning of individual particles. This can be achieved by using the tip of a scanning probe microscope (SPM) as a robot to manipulate the particles [7, 10, 11]. We have previously reported the controlled and precise manipulation of single Au nanoparticles by using dynamic force microscopy in combination with a special-purpose probe control software on several substrates [11, 12]. With this technique, twodimensional patterns can be successfully formed from random arrays of deposited Au particles on a given substrate. However, the mechanisms which underlie these operations are not well understood, and the selection of manipulation parameters is largely based on previous experience and trialand-error. This paper describes a systematic study aimed at improving our understanding of nanomanipulation using DFM. We discuss the experimental procedures and manipulation protocols, simulation studies to guide the experiments, and the experimental results. 1 Manipulation procedures The samples were prepared by depositing 15-nm gold colloidal particles (EM.GC15; Ted Pella Inc.) from aqueous solution on freshly cleaved mica substrates that had been previously coated by a poly-L-lysine film (0.1% aqueous; Ted Pella Inc.). Subsequently, the samples were annealed in air for 3 h at 80 ◦C. The experiments were carried out with an AutoProbe CP R ©AFM (Park Scientific Instruments) operated in noncontact (NC-AFM) and intermittent-contact (IC-AFM) mode. The instrument provides the user with three main parameters for controlling its operation in DFM: (i) the frequency fset of the driving excitation of the cantilever ( fset is usually close to the resonant frequency of the cantilever fres); (ii) the free oscillation amplitude of the cantilever Afree (which is controlled by the amplitude of the excitation driving the cantilever); and (iii) the desired cantilever oscillation amplitude or setpoint Aset. Imaging and manipulation experiments were performed in air and at room temperature using commercially available triangle-shaped silicon cantilevers (Park Scientific Instruments) with integrated conical tips. Hard cantilevers had spring constants ≈ 13.0 N/m and resonant frequencies ≈ 280 kHz, whereas the corresponding values for soft cantilevers were ≈ 3.2 N/m and ≈ 90 kHz. (These are nominal values, supplied by the vendors). Manipulation of the Au nanoparticles was achieved by using the following protocol. First, the sample is imaged. Then, utilizing the probe control software (PCS) developed