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Equipment and Methodology for Multi-Dimensional Scanning Probe Microscopy

Final Report Summary - MDSPM (Equipment and methodology for multi-dimensional scanning probe microscopy)

Executive summary:

The MDSPM project focused on the development of new UHV scanning probe microscopy instrumentation for simultaneous measurements of forces in different spatial directions and access the local electronic structure with highest spatial resolution and sensitivity at temperatures from 4.2 K to 300 K.

A key element of the MDSPM microscope is a focusing Fabry-Perot sensor with bandwidth up to 100 MHz and an unprecedented sensitivity down to 1 fm/sqrt(Hz). The 3μm-diameter focus spot of the Fabry-Perot sensor and its high deflection sensitivity allows the mapping of flexural and torsional oscillation modes of the cantilever. The high bandwidth further allows the detection of higher oscillation modes or harmonics. Thus, the vast range of different types of micro- fabricated cantilevers available commercially allow optimization of the frequency to stiffness ratio and enable the MDSPM to employ the highest quality factor (apart from macroscopic force sensors such as tuning forks) force sensors, making this instrument a truly versatile scanning force microscope. Soft and ultra-small cantilevers give access to the forces in the Attonewton range, while cantilevers with stiffnesses from 10 N/m up to 1000 N/m are optimally suited for atomic resolution work.

Higher oscillation modes of medium stiff cantilevers (several tens of N/m) have recently been successfully used for atomic resolution work with improved resolution and sensitivity. Such medium stiff cantilevers also obtain an excellent sensitivity in the torsional oscillation modes. Operated with ultra-small oscillation amplitudes, these can be used simultaneously with the flexural modes to map lateral forces together with vertical forces, and further allow simultaneous tunnel current measurements with high signal-to-noise ratio. High frequency microfabricated cantilevers also feature highest sensitivity for dissipative tip-sample interactions. These arise from atomistic instabilities or stick-slip phenomena but also from any type of stochastic force fluctuations, i.e. from vibrating molecules or atoms manipulated to unstable surface sites. This opens up a new research area in scanning force microscopy.

The challenges of the here-developed MDSPM instrumentation are its technical complexity requiring new design approaches of many key elements of the microscope. Several time-consuming teething issues had to be overcome. Nevertheless, a highly modular instrument design was accomplished which is of highest importance for the ultimate commercialisation of the SPM technology developed through MDSPM. The instrument design allows the development of various models starting from an instrument capable for STM and tuning fork scanning force microscopy experiments that can be later upgraded and equipped with the Fabry-Perot sensor part to allow the use of microfabricated cantilevers and all advanced scanning force microscopy operation modes developed in MDSPM.

The instruments outstanding performance is proved by:

- a tip-sample gap stability of less than 1 pm even if LN2 was used as a coolant liquid;
- atomic resolution with cantilever oscillation amplitudes below 100 pm was achieved;
- simultaneous scanning force and tunnelling microscope experiments were performed on Au(111) and Si(111)-7x7;
- simultaneous atomic resolution imaging in the fundamental and higher oscillation modes and in the fundamental flexural and torsional mode was achieved.

Two MDSPM instruments were built and installed at laboratories of TCD (Dublin, Ireland) and Empa (Duebendorf, Switzerland). Besides scientific progress, the instrumental development within this project did already generate a positive impact for the two industrial partners (Createc and NanoScan).

Project context and objectives:

The MDSPM project focused on the development of new UHV scanning probe microscopy instrumentation for simultaneous measurements of forces in different spatial directions and local electronic structure with highest spatial resolution and sensitivity at temperatures from 4.2 K to room temperature.

In principle these goals could be reached with a hypothetical SPM instrument making use of a low mass force sensor with a stiffness sufficiently large to avoid a snap-to-contact at small tip-sample distances, and to keep the thermal noise amplitude much smaller than the required stability of the tip-sample distance. Further, a tip-sample gap with sub-picometre stability is mandatory for the implementation of state-of-the-art tunnelling spectroscopy and STM imaging experiments required to access the electronic structure. The simultaneous measurement of forces in at least two directions (vertical and parallel to the sample surface) would require an at least bi-directional force sensor. The small mass of the sensor optimizes the frequency-to-stiffness ratio leading to low thermal noise provided a high quality factor is also obtained. Such a force sensor is required to detect smallest force at reasonable measurement bandwidth. In addition such a microscope must be easy to operate allow in-situ cantilever and sample exchange and the macroscopic positioning of the sample on a scale of several millimetres.

Our approach for an SPM system fulfilling the above criterions was designed for the use of small micro-fabricated cantilevers, although more macroscopic force sensors such as tuning forks could also be used. To profit from the ultra-mall thermal noise of such cantilever at low temperatures, a new deflection sensor with extremely high deflection sensitivity was implemented in a microscope design that further contains piezo-driven positioning units to adjust the relative sensor-cantilever and cantilever-sample distance reproducibly on a millimetre scale with a precision of about 10 nm and a stability better than 1 pm.

Micro-fabricated cantilevers have optimise the frequency-to-mass ratio, can reach a very high quality factor, particularly after baking or at least annealing in UHV to remove amorphous surface layers. Soft and ultra-small cantilevers give access to the forces in the Attonewton range, while cantilevers with force constants from 10 N/m up to 1000 N/m obtain a sensitivity more than two orders of magnitudes poorer than the former but are ideal for atomic resolution experiments. Nevertheless, such stiffer cantilevers still provide force sensitivities far beyond those obtainable by tuning fork and Kolibri sensors.

Cantilevers with force constants above 10 N/m can be approached to an arbitrarily small tip-sample distance because no snap-to contact occurs, provided that the tip is sufficiently sharp. Tip-sample distances smaller than 1nm are also well suited for mapping the tunnel current or performing tunnelling spectroscopy. To allow a simultaneous measurement of forces vertical and parallel to the sample surface we did not use a bi-directional force sensor but decided to implement a uni-directional focusing Fabry-Perot interferometer optical sensor. With this sensor and the attached interferometer system a 3 Â µm-diameter focal spot, a bandwidth up to 100 MHz, and an unprecedented sensitivity down to 1 fm/sqrt (Hz) is obtained. The small focal spot can be adjusted slightly off the long symmetry axis of the cantilever. Then torsional cantilever deflections can be detected. These are a measure for the lateral or frictional force applied to the cantilever. In order to distinguish this lateral from the vertical deflection or force, the cantilever is oscillated simultaneously on its flexural and torsional resonance. The distinction between vertical and lateral force is thus obtained with a uni-directional sensor in frequency space. Tip-sample interaction induced changes of the two simultaneously recorded resonance frequencies are then recorded. The tip position relative to the sample can be kept at a well-defined atomic site, if ultra-small oscillation amplitudes smaller than typical inter-atomic distances are used. Such small oscillation amplitudes however require stiff cantilevers or the use of higher oscillation modes to obtain sufficient stability of the oscillation, particularly when stochastic energy dissipation events for example caused by atomistic position instabilities occur. Then resonance frequencies of several MHz must be detectable requiring a high-bandwidth deflection measurement system, such as provided by our approach.

In order to allow a reliable and easy-to-operate SPM system and make full use of the inherent sensitivity of the micro-fabricated cantilever and the excellent performance of the Fabry-Perot sensor, a modular scanning probe microscope design featuring two 3D-nanopositioning units and new sample and cantilever holder receivers was developed. Details of this instrument are described in the section below. The SPM instrument features an extremely stiff molybdenum body and is attached to an eddy current damping system attached to the bottom of a bath cryostat to decouple the tip-sample gap from outside mechanical noise. Again more details are given in the section below. In addition, a UHV system with a chamber containing the cryostat and the microscope, a preparation and analysis chamber, and a field ion microscope (FIM) chamber was designed and implemented. The FIM allow the determination of the atomic scale structure of the tip.

The main technical specifications stated in MDSPM proposal were (cited from the proposal):

1. Picometre scale probe placement with drift rates of less than 50 pm/hour under LHe operation.
2. Exchangable cantilever sensor with controlled shape, apex composition and structure made possible by in-situ forming capability and characterisation by FIM.
3. Unprecedented range of force sensitivities (100 fN to 10 nN ) with simultaneous sensitivity for normal and lateral forces.
4. Stable tunneling condition during AFM operation with full STS spectroscopic capabilities.
5. Cantilever oscillation amplitudes less than 10 pm possible.
6. Multiple modulation capability that records simultaneous picometre scale maps of the force field, LDOS (dI/dV, d2I/dV2), and relaxation spectroscopy (dI/dZ, d2I/dZ2 ).

Experimental results obtained within the proposal demonstrate that point 1, 3, and 4 were obtained. Experiments have further proven that that cantilever holders can easily be exchanged and that the apex of STM tips can be analysed with the installed FIM. Atomic resolution imaging with ultra-small cantilever oscillation amplitudes below 100 pm was achieved and oscillation amplitudes smaller than 10 pm are shown to be achievable. So points 2 and 5 are at least partially fulfilled. The tested performance of the instrument further allows the conclusion that point 6 can be obtained as well, but the corresponding experiments have not yet been performed.

Several teething issues have severely delayed experimental progress. Among these were issues with the MDSPM instrument itself, with the interferometer system and with the software and electronic control system.

All technological problems of the microscope have now been solved. The last one, which caused a failure of the Empa MDSPM z-walker at low temperatures, was actually resolved during writing this report. Only two of the many problems were really caused by an non-optimal design; all other problems can be considered as true teething issues. Unfortunately, in the proposal no time to resolve teething issues was planned clearly because the consortium feels that such a proposal would fail acceptance, - although this would better state the reality in a technological development project.

The performance of the interferometer is still not totally satisfactory, although work-around strategies have been defined and reported in detail. A final solution is expected when an external cavity stabilized laser diode is delivered to Empa for further testing.

The electronic control system was also a major time sink for the Empa and TCD staff working on the microscopes suffering from various software bugs leading to a destruction of many tips. Considerable time was further invested into debugging software rather than into true experimental work. The control system now operates stably, but its performance still falls slightly short compared to competitive systems available on the market. Particularly because important parts of the software are still missing or have only been implemented in a very rudimentary form. Further limits arise from the internal value update cycle time, which is limited to about 5 kHz limiting the performance of the instrument because the minimum z-feedback reaction time is larger than 2 ms. In-spite of all these issues, the software now operates stably, and some of the implemented concepts in particular the safety conditions surpass any other solution on the market. Moreover, NanoScan agreed to deliver the missing parts, i.e. 3D-spectroscopy and atom manipulation after the completion of the project.

In-spite of the major technological difficulties and other delays experienced in the course of the proposal work, a major part of the proposed key experiments could still be accomplished.

These were (citation from the proposal):

1. atomic resolution imaging with 0.01 nm oscillation amplitude possible;
2. atomic resolution imaging with higher cantilever oscillation frequencies possible;
3. atomic resolution imaging with torsional cantilever oscillation modes possible;
4. manipulation of atoms and molecules performed;
5. simultaneous measurement of tunnelling current and force possible;
6. measurement of dissipation of energy from flexural and torsional modes into the sample possible.

Apart from points 4 and 6, all points have been addressed within project. Point 1 is not yet fully obtained. Nevertheless, with the proven performance of the instrument, the research teams of TCD and Empa have no doubts that the other key experiments will soon be completed successfully.

Project results:

The most important output of this project is the design and fabrication of two MDSPM systems installed at Empa (Switzerland) and TCD (Ireland). Both systems were built and assembled at Createc in close collaboration with Empa scientist after the microscope technology developed at Empa has been transferred to Createc. The electronic control system was developed and installed by NanoScan, again profiting from technology developed at Empa, e.g. the PLL or the interferometer system. Software concepts have been designed by scientists at TCD and Empa and transferred to NanoScan for implementation.

Review on developed instrumentation

UHV system:

The UHV System is a two-chamber system with an integrated valve for chamber separation based on a previously existing design of the Createc LTSTM.

Changes relative to the latter system included, a shift of the top cryostat flange out of the long axis of the system, and an additional chamber for the field ion microscope (FIM). One of the two main chambers serves as a preparation and analysis chamber, while the other contains the cryostat with the attached SPM. Both main chambers are pumped by ion-pumps that include Ti-sublimation. The preparation and analysis chamber is further attached to the load-lock. A sample or a cantilever holder entered via the load lock and its magnetic linear manipulator is transferred to the cryo-manipulater via a gripper / screw-driver manipulator.

The cantilever and sample holders are all based on the same principle allowing the design of various types of sample holders. The key elements are four massive contact plates that, apart from a single thread-hole, can be machined by laser cutting at low cost. A ceramic plate platform without threads then connects the four contact plates to form a mechanically stable entity that keeps sufficient flexibility to allow four contacts. On the top of the ceramic platform various types of cantilever or sample holders can then be constructed.

The sample or cantilever holder transport inside the UHV system is performed by a r-xy-z cryo-manipulator. The rotation (r) allows the rotation of the sample towards the Knudson cells mounted at the bottom of the preparation chamber. In the MDSPM chamber, the sample or cantilever holder can be transferred to the SPM using a second gripper / screw-driver manipulator. The latter is also used to open and close the shutters of the cryostat shields. Samples or cantilevers are then placed without any force into the corresponding receivers in the microscope. Once the holders are placed the screwdriver is used to turn the locking screw that pushes a spring to the bottom of the holder and clamps the latter to the receiver. Then a sample or cantilever holder is rigidly fixed and also electrically contacted. A sample or cantilever holder transfer from the load lock into the microscope takes only about 2-3 minutes.

Two UHV systems were built one for Empa and one for TCD. In the TCD system, the FIM chamber is not inline and attached to the preparation chamber. Instead it is attached to the SPM chamber.

Cryostat:

A bath cryostat built by Cryovac to specifications of Empa and Createc is mounted to the top flange of the SPM chamber. Two rotary shields are attached at the bottom of the cryostat. The outer of the shields, the LN2-shield is attached to a tube-like LN2 container that screens the inner He-bath from room temperature radiation. The inner shield attaches to the Cu-bottom plate of the LHe-tank. This bottom plate also serves as a heat sink for the low thermal conductivity wires running down the outside of the LHe-tank ending in a in various electrical connectors. These connect to the various functional units of the microscope. Each functional unit of the microscope has its own bundle of electrical wires ending in one electrical connecter, allowing a simple demounting of each unit from the microscope. The latter is spring suspended from the bottom of the LHe tank. At the bottom of the microscope an Eddy current damping system is attached. The cylindrical LN2- and LHe shields have various openings for sample / tip viewing and exchange and evaporation to the sample or tip. These openings can be opened or closed by rotating cylindrical shutters attached to the inside of the LN2- and LHe-shields. The LN2-shutter is located in the inside of the LN2-shield such that it can push the LHe-shutter which is located on the outside of the LHe-shield. 4 different shutter positions exist. The LN2-shutter rotation is achieved by pushing the shutter through a slit in the LN2-shield.

Microscope:

The microscope consists of a tubular molybdenum body that contains the cantilever holder receiver and the Fabry-Perot (FP) and sample (S) positioning units. The FP unit allows a 3D-adjustment of the FP optics relative to the cantilever over 1 mm along and perpendicular the long cantilever axis and about 10 mm towards and away from the cantilever. The FP unit further contains the w-piezo that can fine-adjust the FP optics to cantilever distance (cavity length). The S-unit adjusts the sample relative to the spatially fixed cantilever on a scale of 4 x 4 mm2 in the xy-plane and about 10 mm towards and away from the cantilever. The S-unit further contains the scan-tube to the top of which the sample holder receiver is mounted. It is noteworthy the S positioning unit is designed to have the smallest mechanical loop in the approached state. Moreover, it is highly symmetrical in the xy-plane and the materials and geometry are chosen to reduce and thermally compensate the mechanical loop. This leads to an extraordinary low drift rate. The mechanical rigidity decouples the tip-sample gap from external mechanical vibrations.

The microscope design was further governed by commercial considerations:

1. The microscope is modular allowing a rapid exchange of all parts for servicing.
2. The microscope can be sold as a simple STM only containing the S unit and the spatially fixed cantilever or tip holder receiver. Force microscopy experiments would then still be possible if tuning forks are used as force sensor.
3. The microscope can later be upgraded with the FP positioning unit. The FP unit then allowing the detection of the cantilever deflection.
4. The FP unit can be equipped with a FP optical sensor or with a carrier for a parallel polished fibre ferule. The parallel polished end face of the fibre and the cantilever would then form an interferometer cavity. This cavity will certainly have a lower finesse than the FP optics to cantilever cavity, but may be sufficient for most experiments and rather inexpensive.

Clearly a sales and marketing strategy still needs to be defined.

FP sensor:

In the project, a new five-lens, chromatically corrected FP sensor has been designed and several three versions of this optics (for the Empa prototype LTSFM, MDSPM, and TCD MDSPM) have be built. All these sensors operate well at room temperature, LN2 and LHe temperature.

Interferometer system:

At the start of the project, only an interferometer system based on Hug's former developments at the Uni Basel was available. In 2002-2004, Hug decided to move away from an all-fibre-optical set-up to a more integrated solution. OECA has then custom-fabricated our interferometers. These feature a 785 nm laser diode with an optical insulator, a beam splitter, a signal and a reference photodiode, a blocking screw to reduce the laser power, and a fibre coupler.

Within the project a new interferometer system with the following features was developed:

1. Replacement of all-in-one solid-state interferometer previously designed and built to Empa order by an all fibre components interferometer. This provides a higher flexibility for testing and facilitates the exchange of broken components.
2. Second laser with 635 nm wavelength for optical excitation.
3. 785 nm laser with increased power output and which is less sensitive to back-reflections.
4. Replacement of the beam splitter by a all-fibre-optical circulator for improved transmittance from the laser to the instrument and from the instrument to the photo diode.
5. High frequency external modulation input for both laser diodes.
6. Modular photodiode receiver that allows later adaptions, i.e. implementation of avalanche photodiodes for bandwidths up to 100 MHz.

Both, the old and here-developed new interferometers use the same laser diode. In the new, improved interferometer the optical power into the fibre is more than 30 times larger (although the new system includes a WDM to mix the light of the second (635 nm) laser diode into the fibre. In addition, for the same outgoing power, the transmission of the back-reflected light to the signal photodiode is four times higher in the new system.

The new interferometer system further allows the optical excitation of the cantilever. Mechanical excitation at frequencies between 100 kHz and 1 MHz often generates a series of resonance peaks where most of them arise from resonances of the cantilever holder or SPM body. In some cases, these resonances peaks can become larger than the cantilever resonance making it difficult to distinguish the latter from the resonances of the microscope. Optical excitation then allows a clear identification of the correct (cantilever) resonance. Optical excitation is performed by modulation of the power of the 635 nm laser diode. This generates a thermo-mechanical force driving the cantilever excitation.

One of the main design goals of the new interferometer system was the improvement of its dc-stability. With the old interferometer system the interferometer signal typically drifted up and down on a scale of a few Hz. This leads to a corresponding instability of the operation point on the interference curve, because the w-feedback keeps the dc-interference signal constant. The instability of the operation point then causes a corresponding fluctuation of the local slope of the interference curve, and hence a fluctuation of the interferometer sensitivity. The measured cantilever oscillation amplitude hence fluctuates. The amplitude feedback of the PLL then adjusts the excitation amplitude to keep the measured cantilever oscillation amplitude constant. This however induces fluctuations of the real cantilever oscillation amplitude. These lead to instabilities of the tip-sample distance and sensitivity of the cantilever for tip-sample interaction forces. Several solution strategies have been developed.

1. A lock-in technique can be used to keep the operation point at a pre-selected slope of the interferometer signal: The w-piezo (cavity) is modulated with a suitable frequency, i.e. 1 - 4 kHz (higher than the bandwidth of the w-feedback and higher than the bandwidth of the PLL phase loop). The corresponding output of the interferometer signal is then feed to a lock-in. The w-feedback is then used to keep the amplitude output of the lock-in constant. The operation point then follows the fluctuations of the DC-level of the interferometer signal, such that the slope at the operation point remains constant.

Note, that this method requires an additional lock-in amplifier (not included in the Nanoscan control system) and requires rewiring the input signal of the w-feedback to use either the interferometer signal or the output of the lock-in amplifier.

Empa has successfully tested the slope-feedback scheme. The best stability was obtained if the w-piezo modulation amplitude was chosen equal to the cantilever oscillation amplitude. Then both oscillation amplitudes are equally affected by a dc-shift of the interferometer signal.

Note that to date no software concept has yet been developed to select between a w-feedback operation on the interferometer dc-signal or on the output of the w-modulation lock-in.

2. Alternatively improved laser sources such as external cavity stabilised lasers could be used. A detailed evaluation is presently underway. Note that a change of the laser diode can be easily performed in the new interferometer system. A new cavity-stabilised laser diode was recently ordered by Empa and will be delivered in a few weeks.

Electronic control system and software:

A prototype improved PLL electronics consisting of an analog lock-in amplifier to measure the phase and the amplitude of the cantilever oscillation signal and three digital analog converters was designed at Empa. The technology was transferred to NanoScan who improved various parts of the prototype design and developed a card for the PXI chassis and implemented the software to control two PLLs.

Various software concepts were developed by Empa and TCD and again transferred to NanoScan for implementation. Among these concepts are the following:

1. New z-feedback concepts:

The determination of best P and I values for the z-feedback are particularly problematic, if the measured tip-sample interaction is non-linear. This is the case for the tunnel-current that depends exponentially on the tip-sample distance, but also for short-range interaction forces that show a similar distance dependence in the attractive interaction regime. Smaller distances will lead to an increased z-derivative of the measured interaction and consequently to an overall increase of the feedback loop gain. Thus P and I values must be adapted to reduce the feedback gain to avoid feedback oscillations. First attempts to perform STM with Nanoscan's control system were not successful. The feedback parameters were difficult to set and small changes of the setpoint current or applied bias regularly led to tip crashes. Empa performed extensive tests that conclusively showed that the feedback step answer time was 2 ms, considerably too slow to allow stable feedback for an exponential tip-sample interaction dependence, i.e. during tunnelling. Improved concepts for a feedback on log and other similar modes were developed and timely implemented by Nanoscan. Nanoscan was further able to improve feedback speed by about a factor of 2. The about 0.8 ms reaction time is however still a bottleneck for the MDSPM's performance. A feedback speed better than 0.1 ms would be required to fully profit from the high gap stability of the MDSPM and the greater than 2.5 kHz resonance frequency of the MDSPM scanner.

2. Break or safety conditions:

These have been further extended and successfully implemented by NanoScan. The break conditions proved to be extremely useful for the protection of the tip during nc-SFM operation. Moreover, the break-conditions is a truly unique piece of software not existing in control systems of competitors!

3. Master window:

In order to select coordinates to perform various functions such as zooming, moving scan centre, moving tip to a position for manipulation, spectroscopy, force-distance measurements, the latest recorded measurement data, or data from a file, can be loaded into a master window from which all selections can be made. NanoScan has implemented a minor part of this concept as a part of their atom manipulation software package. Moreover, the previously existing software allows the selection of single points or lines for 1D or 2D spectroscopic measurements.

4. Multi-dimensional spectroscopy:

The concept of multidimensional spectroscopy was based on the currently installed 2D scan window.

Unfortunately, this software concept is not yet implemented. In the mean-time a previously existing piece of software, the general scan window can be used to define 1D and 2D-scans of most parameters available by the software.

More details, - particularly about the handling of the various feedback loops required for non-contact SFM operation during multi-dimensional scans are described in the corresponding document. Unfortunately the above concept is not yet integrated.

5. Atom manipulation:

A concept for atom manipulation was developed by Empa in collaboration with scientists from NIST in Gaithersburg US and scientists from TCD. Again the concept has been transferred timely to Nanoscan for implementation. Unfortunately, Nanoscan has only implemented a preliminary version thereof, which does not yet allow the recording of the data during the manipulation steps.

Review on obtained results

Tip-sample gap stability:

The tip-sample gap stability is best tested by recording a spectrum of the tunnel-current with a slow z-feedback to correct for drift. The z-feedback speed must be set as slow as possible. Then the recorded tunnel current spectrum can be analyzed above the z-feedback cut-off. Calibration of the tunnel current spectrum can then be done by modulation of the z-position at a frequency higher than the cut-off.

Although various tunnel spectra have been recorded, a systematic noise calibration was not yet performed. Instead the noise can estimated from STM data on Au(111) performed with the LTSFM at 77 K at Empa.

We found that the atomic corrugation is between 2 and 4 pm with a noise less than 1 pm. This proves the extraordinary stability of the tip-sample gap of the LTSFM instrument, even when bubbling noise of the LN2 is present. Similar data recorded with the MDSPM suggests that the MDSPM even surpasses the performance of the LTSFM.

Another important issue in SPM is drift. Thermal drift in the xy-directions can be reduced by a highly symmetrical microscope design. If the sample xy-positioner is in its middle position the microscope is nearly mirror symmetric in respect to a plane defined by the tip-sample vector and the long axis of the cantilever. Further the microscope has a high symmetry along the plane again defined by the tip-sample vector and the direction across the cantilever long axis. This reduces drift in the xy-direction to a minimum. Along the tip-sample direction the drift is reduced by a thermally compensated microscope design. Measurements performed at 77 K have revealed that tunnelling with a deactivated z-feedback was possible over 36 minutes.

Atomic resolution imaging with ultra-small oscillation amplitudes:

High quality resonance curves have been recorded for cantilever oscillation amplitudes down to 0.01 nm with the MDSPM system at Empa. However, during measurement performed with the Empa MDSPM system, the smallest amplitudes have not yet been used for the following reason:

The Empa MDSPM suffered from a defect of the z-motor that prevented an approach of the sample to the tip at lower temperatures. Hence to date, the instrument was used solely at room temperature permitting a reliable approach. In the last two weeks, fortunately the problem with the z-motor could be traced back to mechanical parts being outside of the stated tolerances. In the last three weeks, these remaining teething problems were resolved. We expect the Empa MDSPM will finally be operational at low temperatures.

To date, the smallest oscillation amplitude was selected to be 0.3 nm, to remain considerably above the thermally driven oscillation amplitude. The following images have been acquired on a 7x7 reconstructed Si(111) sample with current-feedback control and simultaneous recording of the frequency shift and oscillation amplitude signals.

These prototypical data show that imaging with small amplitudes (here 0.3 nm) is possible even when working at room temperature. We expect that at LHe temperatures where the thermal noise amplitude will be decreased by about a factor of 8, imaging with the same quality is possible with oscillation amplitudes smaller than 0.1 nm.

Small amplitude operation was also demonstrated by MDSPM system at TCD. The sample used was once again the Si(111)-7x7 surface. An untreated Si cantilever probe was used in all experiments. The amplification range before the PLL was carefully chosen so as to reduce the level of digitization noise and to increase the phase sensitivity. All measurements were made using the first harmonic flexure mode with a resonance frequency f1 = 1 824 778 Hz with the excitation amplitude set to 90 pm. While the 7x7 unit cell is resolved the adatom details within each cell cannot be imaged with the Si cantilevers used in these experiments. The horizontal streaks visible in each image are due to a passing train.

Simultaneous STM and SFM measurements:

At Empa simultaneous measurements in STM and AFM mode have be performed in two different operation modes. The frequency shift and the excitation signal were simultaneously recorded. It is noteworthy that above the ad-atoms a less negative frequency shift was recorded. This indicates that the tip was either in a non-reactive state or the tip-adatom distance was too large to generate a short-range attractive force that would lead to a more negative frequency shift. In a case where no short-range attractive bonding force appears on the ad-atoms only van der Waals and electrostatic forces are present. These become less attractive with increasing tip-sample distance. Hence above the adatoms a less negative frequency shift is generated.

A similar performance was also recorded with the MDSPM system shipped to TCD. To demonstrate good resolution a diamond probe tip was used to prevent the formation of an unreactive probe surface due to oxidation (which occurs easily in case of Si).

Multidimensional AFM simultaneous measurement of vertical and lateral forces with atomic resolution:

A key MDSPM experiment, not possible in conventional tuning fork or qPlus systems, is the ability to simultaneously measure vertical and lateral forces. It is our expectation that simultaneous vertical and lateral mode imaging will become routine and when employed with an appropriately prepared probe tip will lead to atomic resolution imaging and multi-dimensional dissipation mapping or surface and molecules.

A first experiment was performed with a Si cantilever for which the PLLs were set to the first harmonic flexure mode f1 and the torsional mode ft, which occur at 1 824 778 Hz and 2 270 433.5 Hz, respectively. The sample chosen was the Si(111)-7x7 surface. The topographic z-feedback condition was set by specifying a frequency shift in the flexure channel df1, while the change in the torsional frequency dft was simultaneously recorded.

Imaging of molecules:

With the Empa LTSFM prototype system dibutyl-sulfide molecules were deposited from the liquid phase onto the Au(111) surface. The molecules were first imaged by STM at 77K.

So the molecule appears as an overlay of three linear objects with two main intensity lobes, i.e. as a six-lobed object. A careful analysis of the data revealed that some molecules do not occupy all of the three states with the same probability. In addition to the STM data shown below, data has also been acquired with the SFM operated in a constant frequency shift imaging mode. Hence dibutyl-sulfide molecules were imaged consecutively with STM and SFM.

Controlled nanoscale contacts:

Although it was not possible to perform the metal inking experiment to control the metal coating on the AFM probe as originally envisioned, experiments were performed in which the probe was controllably contacted the surface during which the vertical deflection of the probe and the tunnelling current was simultaneously measured. STM is a displacement-controlled technique in which I-z curves are available over a large z range due to the inherent high stiffness of the STM probe. In contrast MDSPM measurements enable us to begin to examine the details of the contact mechanics. Clearly there is no region where the probe is in stable tunnelling with the surface, i.e. a region where it increases exponentially as z is decreased. Instead, the probe jumps into contact before an extended region of stable tunnelling. However, upon retraction the current decreases over a wide range, showing a series of kinks and steps. This behaviour is due to the mechanical manipulation of the contact, which changes from compressive to tensile loading during the approach and retraction. The panel on the right shows a simple JKR model of contact between a rigid bead and a deformable surface. Clearly, the MDSPM offers a possibility to study the contact mechanics and energy dissipation involved.

Summary:

The MDSPM project has suffered from major technological difficulties that have caused severe delays. Nevertheless, new advanced scanning force microscope technology was developed and transferred to Createc who has built two MDSPM systems and installed these at Empa and TCD. NanoScan delivered the electronic control system including software, also based on concepts developed by Empa and TCD.

Both MDSPM systems are operational now, although a last teething issue, the improvement of the stability of the laser source of the interferometer system still has to be improved. Moreover, major software development and debugging of the software still need to be done. Nevertheless, Empa and TCD could demonstrate that the functionality of the MDSPM meets all requirements defined in the project. First promising scientific results were obtained on Dibutyl molecules deposited on a Au(111) surface. We expect that high impact scientific results will be obtained within one year.

Potential impact:

The main goal of the project was the development of new scanning force microscope technology surpassing existing state-of-the art instrumentation. Although two MDSPM systems were built, installed at Empa and TCD, and used to demonstrate the main scientific objectives of the project, key scientific break-through results have not yet been obtained.

Nevertheless, promising new scanning force microscope technology has been developed and transferred to Createc. The developed instrumentation technology can be considered to be close to a final product. On the long term we expect that Createc will be able to sell such MDSPM systems once break-through scientific results not possible with current state-of-the art SFM technology are obtained. On the short term, Createc will profit from considerable experience gained in piezo-motor technology and hand-on operational experience with various scanning force microscopy operation modes, also usable for improvements of their existing product line. Empa and TCD have agreed to deliver key results and operational experience even after the completion of the project to Createc to facilitate a future market introduction of the MDSPM system.

NanoScan has strongly profited from the input of Empa and TCD for continued development of their electronic control system and software financed through the MDSPM project. The new double PLL system and new software such as the break conditions and improved z-feedback schemes are mandatory developments also for all other products of NanoScan.

Empa and TCD finally profit from the availability of advanced UHV, low temperature scanning force microscopy equipment installed in their laboratories. These instruments serve as a nucleation point of new scientific projects that would not be possible without this advanced MDSPM instrumentation.

As anticipated in the original proposal MDSPM has begun to shape future research programme opportunities and the interactions of the partners with industry. At Empa a project financed by the Swiss National Science foundation relying on the MDSPM system has recently been started. CRANN in turn will exploit the capabilities of MDSPM in their recently funded FP7 MOLARNET programme that seeks to demonstrate the operation of molecular quantum cellular automata. This programme, which depends on the controlled shuttling of electrons between sites due to electrostatic repulsions would be unthinkable without the MDSPM capability.

Industry too has become interested in the benefits provided by the MDSPM. A new research contract with Intel will explore the operation of nanoelectromechanical (NEMs) switches as potential hard-off switches for reduced power consumption in mobile device applications. Controlled MDSPM contact mechanics experiments will enable the team to address the pull off mechanics and whether there is sufficient elastic energy stored in the closed NEMs device to overcome adhesion at the contact, i.e. the minimise losses in device operation. This industry funded programme, and others like it, represent important inward investments into the EU, which not only recognizes the world leading expertise in SPM technology developed through MDSPM, but strengthens the existing investment of companies in terms of footprint and jobs within the EU. Such engagements are essential in this global open innovation research environment where the development of MDSPM will allow research teams within the EU, through access to commercially produced MDSPM instrumentation, to collaborate with industry (small medium enterprises and mulit-nationals) both within and beyond the EU, so as to insure the Europe is the favoured go-to partner for future collaboration, feasibility studies and product innovation.

Public website address:

The project team has installed an official website of the project since the beginning of the project. You can open the page on http://www.mdspm.eu This address will not be given up after project. As soon as the MDSPM machine has improved so far, that the expected distribution can be done this site will be the master page of the MDSPM-machine, linked to the existing homepage of CreaTec. The improved website than will be done in TYPO3.

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