SPM 150 Aarhus with KolibriSensor TM

Transcription

1 Surface Analysis Technology Vacuum Components Surface Analysis System Software Computer Technology SPM 150 Aarhus with KolibriSensor TM Technical Notes Using the SPM 150 Aarhus with KolibriSensor TM, SPECS combines its proven extreme stability of the Aarhus design with state-of-the-art dynamic atomic force microscopy (AFM) techniques. The KolibriSensor TM, based on the piezoelectric length extension resonator (LER), enables the scientist to study every surface regardless of its conductivity at the atomic scale. The SPM 150 Aarhus can be operated in the scanning tunneling microscopy (STM) or non-contact atomic force microscopy (NC-AFM) mode, or in a combination of both modes with simultaneous but independent measurement of force and tunneling signals. Features KolibriSensor TM : High frequency (~ 1 MHz), high Q (> 10,000 at room temperature), and very stiff (k~540,000 N/m) piezoelectric force sensor SPECS Nanonis Control System with highly accurate Phase Locked Loop (PLL) Operation of the AFM with sub-nanometer oscillation amplitudes for enhanced detection of short-range interaction between tip and sample Clear separation of force signal and tunneling current via separately contacted tungsten-tip On the fly switching between STM and AFM feedback modes In-situ sensor preparation by ion beam sputtering Temperature range K

2 Principle of NC-AFM The introduction of the atomic force microscope (AFM) provided a scanning probe microscopy method capable of imaging all surfaces regardless of their conductivity by measuring forces between a nanoscopic tip and the surface. The dynamic scanning force microscope operated in non-contact mode (NC-AFM) in an ultra-high vacuum environment has been developed into a versatile surface science tool for the characterization of insulating and conducting materials at the atomic scale. In dynamic scanning force microscopy a nanoscopic tip mounted on an oscillating quartz rod is scanned laterally over a surface. Simultaneously the quartz rod is excited to vibration at its resonance frequency. The change in resonance frequency of the excited tip upon approach of the tip towards the surface is used as the feedback signal for the tip-surface interaction. Figure 1: The graph shows the typical dependence of the tipsample interaction force as a function of distance z and the resulting detuning f versus z dependence. Images courtesy of S. Torbrügge, Dissertation 2008, University of Osnabrück. A schematic drawing of the tip-sample interaction force is shown in Figure 1. The interaction between the oscillating tip and the surface can be separated in four parts: (O) free oscillation region where the oscillation is not influenced by the tip-sample interaction. (I) long range interaction governed by the macroscopic tip geometry, (II) short range interaction determined by the nanoscopic tip geometry and chemistry and (III) short range repulsion. In total a superposition of attractive and repulsive forces results in the tip-sample force. (b) Typical characteristics of the detuning f as a function of tip-sample distance z. To obtain a topographic image the detuning is kept constant at a given set point f set. Atomically resolved images are usually taken in the short range tip-sample interaction region (II).

3 Technical Design Figure 2: Schematic drawing of the quartz KolibriSensor TM. The sensor oscillates with amplitude A perpendicular to the sample surface. Figure 3: Typical resonance curve of the SPECS KolibriSensor TM recorded in a vacuum at room temperature. SPECS offers the KolibriSensor TM, a new type of force sensor based on a quartz length extension resonator (LER). This sensor design combines a high spring constant k and excellent signal-to-noise ratio necessary for stable operation of the sensor at sub-nanometer oscillation amplitudes. The sensor consists of an oscillating rod supported by two arms at the center. Each side of the rod is coated with an Au electrode. The resonance oscillation is excited by applying a sinusoidal voltage U EXC on one of the two electrodes of the rod. When the rod is extended or contracted, electronic charges due to the piezoelectric effect are induced on the surface of its side walls and detected as a current I OSC from the other electrode on the rod. For a clear separation of oscillation current I OSC and tunneling current I tunnel the metallic tip is electrically insulated from the LER and contacted to a third electrode by a wire. The sophisticated SPECS KolibriSensor TM design with highly skilled manufacturing techniques provides high quality oscillators with Q-factors above 10,000 at room temperature in vacuum at a resonance frequency of about 1 MHz. The high resonance frequency together with the high spring constant k ensure a fast response of the KolibriSensor TM but require a highly accurate Phase Locked Loop (PLL) to perform NC-AFM measurements due to the low frequency shifts in the range of a few Hertz. The Nanonis Control System with its high precision PLL and the proven perfectpll TM setup tool meet these requirements, enabling atomic resolution performance of the KolibriSensor TM during NC-AFM operation. Due to direct electromechanical coupling the power dissipation Γ of the oscillation can be measured directly by Γ = U exc I osc With no tedious calibration required, this is a very powerful advantage of piezoelectric oscillators in dynamic force microscopy. Furthermore the KolibriSensor TM design facilitates in-situ ion beam sputtering of the tip enabling repeated sharpening of the sensor tip on a daily basis.

4 Results All displayed data have been taken with no external dampers on the UHV system. 5th floor of SPECS building (severe low frequency noise) Mechanical pumps running within lab space Heavy duty labor inside and surrounding the lab space All displayed images represent raw data with no filtering or smoothing applied.

5 NC-AFM Imaging on Si(111)(7x7) z f I t Γ Figure 4: Atomic resolution NC-AFM image of the Si(111)(7x7) surface in the constant detuning ( f) feedback mode. Topography z, detuning f, tunneling current I t, and the dissipation Γ of the oscillation are recorded simultaneously. image size: (7 x 7) nm 2, 256 x 256 pixels resonance frequency: f res = 996,035 Hz imaging set point: f = -0.9 Hz oscillation amplitude: A = 200 pm contact potential difference: U CPD = 0.25 V imaging speed: 10 lines/s

6 NC-AFM Imaging on Si(111)(7x7) z f I t Γ Figure 5: Atomic resolution NC-AFM image of the Si(111)(7x7) surface in the constant detuning ( f) feedback mode. Topography z, detuning f, tunneling current It, and the dissipation Γ of the oscillation signal are recorded simultaneously. During imaging from bottom to top tip-changes (exemplarily marked by arrows) occur, primarily affecting the tunneling current while the f signal and thus the topographic image remain unperturbed. image size: (6 nm x 5 nm), (256 x 210) pixels resonance frequency: f res =981,524 Hz imaging set point: f=-0.18 Hz oscillation amplitude: A=600 pm contact potential difference: U CPD = 0.55 V imaging speed: 2.5 lines/s

7 Spectroscopy on Si(111)(7x7) Figure 6: Combined spectroscopy measurement of f and tunneling current versus distance z on the Si(111)(7x7) surface. Oscillation amplitude A = 400 pm. The steep drop in f(z) at about z=-0.3 nm indicates the onset of the short-range interaction regime. Combined spectroscopy experiments enable the detailed study of the onset of the short range interaction and tunneling current. Figure 7: Combined spectroscopy measurement of f and tunneling current versus applied bias on the Si(111)(7x7) surface. The maximum of the f(z) parabola indicates the minimum of the electrostatic force which is equivalent to the contact potential difference (U CPD ) between tip and sample.

8 NC-AFM Imaging on Insulating KBr(001) z f A Γ Figure 8: Large Scale topographic image of the insulating KBr(001) surface in the constant detuning ( f) feedback mode. Topography z, detuning f, oscillation amplitude A, and the dissipation Γ of the oscillation signal are recorded simultaneously. image size: (350 nm x 350 nm), (512 x 512) pixels resonance frequency: f res = 981,532 Hz imaging set point: f = -1.2 Hz oscillation amplitude: A = 400 pm contact potential difference: U CPD = 0.35 V imaging speed: 2 lines/s

9 NC-AFM Imaging on Insulating KBr(001) z f A Γ Figure 9: Atomic resolution AFM image of the KBr(001) surface in the constant detuning ( f) feedback mode. Topography z, detuning f, oscillation amplitude A, and the dissipation Γ of the oscillation signal are recorded simultaneously. The cubic unit cell is highlighted by a black square in all images. image size: (4 nm x 4 nm), 256 x 256 pixels resonance frequency: f res = 995,035 Hz imaging set point: f = Hz oscillation amplitude: A = 100 pm contact potential difference: U CPD = 2.03 V imaging speed: 3 lines/s SPECS GmbH Surface Analysis and Computer Technology Voltastrasse Berlin GERMANY Phone: Fax: support@specs.de