Pulsed Laser Ablation and Pulsed Laser Depostion
The technique is based on a small turbomolecular pumped vacuum vessel with access for the laser beam (1064, 532, 355 and 266 nm from a Q-switched Quantel Brilliant b laser), an optical port for stereomicroscopic visual inspection of the ablation sample, an electrostatic deflection system to discriminate between ablated neutrals and ions, a manually microtranslated (XYZ) mask holder, and a liquid nitrogen cooled cold finger cryostat. The available setup is reported in Figure 1.
Figure 1 – The Pulsed Laser Depostion (PLD) equipment has the possibility to micrometrically translate the sample or the shadow mask in order to achieve micropatterned depositions, and to ablate biological samples from frozen solutions. Other micro or nanopatterned deposits can be obtained by e-beam lithography (A. Gerardino, CNR-IFN).
The possibility to steer ionized atoms and molecules out of the ablated neutral beam and address ions on a surface along a trajectory at right angle with respect to the ablation direction allows accomplishment of extremely uniform growth at very low growth rates. This has produced recently very uniform and thin metal layers. Figure 2 reports a lithographically fabricated pattern of platinum squares as imaged by SEM (left) and by AFM (right)
Figure 2. SEM image on the left, AFM detail on the right
The Pt layers we obtain by PLD are extremely thin and uniform. In this AFM scan we have an atomically uniform, 9 Angstrom thick layer, while Figure 3 reports a detail of one such square showing the step height and thickness.
The associated histogram reports the height distributions of the topographic image for both the uncoated native silicon oxide on silicon and for the platinum coated part.
Figure 3
We can see that the step height is 9 Angstrom, while the distribution width of the substrate and of the deposition is identical, showing that the Pt coat does not contribute to the substrate roughness. The same apparatus is used for micropatterned metal deposition (Figure 4) and “soft” biomolecule deposition (Figure 5) using microtranslated shadow masks. Furthermore, visual microscopic control of the laser spot on the target allows quick and easy fabrication of micrometric shadow masks (Figure 6). Platinum micropatterned PLDs on Si as imaged by SEM on the left. On the right, selectively coloured luminescence from chromophore tagged antigens reacting with Pulsed Laser Depositions of their specific antibodies (confocal microscopy). Pulsed Nd-YAG laser drilled shadow mask on 100 micron thick stainless steel blade.
Figure 4 Figure 5 
Figure 6
STM under SEM observation with laser excitation of the specimen
A home made piezoscanner, designed for xy scanning of the sample, is hosted on the sample stage of a modified Cambridge S200 scanning electron microscope, while the STM tip is positioned on the desired feature under study by a ρ,θ,φ nanomanipulator. This configuration is the most suitable to minimize mechanical vibration of the STM; alternatively, a particularly low weight scanner is mounted on the nanomanipulator while the sample is fixed. Laser beams can be directed to the interaction zone through an optical port at the back of the SEM chamber, which also serves for microscopic observation of the laser beam alignment on the STM junction. Directing the Laser beam onto the tip allows tip enhanced localised laser nanostructuring (ablation or multiphoton deposition) and localised spectroscopy.
A scheme of the STM-SEM facility is reported below (left). The images on the right show an optical microscope view of the laser irradiated STM tip on the sample, and an SEM scan of the STM tip operating to deposit material on the tips of a couple of lithographic nanoelectrodes fabricated by A. Gerardino at the CNR-IFN electron beam facility. 
This facility is particularly useful for precise positioning of the tip on the specific features or devices to be analyzed or operated upon. It allows STM electronic spectroscopy to be effected on features previously easily located on the basis of the difference in secondary electron emission. It allows a prompt characterization of both tip and sample surface damage. It supplies a first easy characterization of surface modifications, like nanometric pit formation or inorganic material depositions, when the sample is irradiated by laser beams. Finally it gives us the possibility to effect much shorter, and as a consequence slower and less damaging, STM scans for a complete characterization of the sample.
Nanomanipulated Piezoelectric Dynamometers (AFM and Piezoresistive Cantilevers)
Within the general issue of nanofabrication in our laboratory we are interested in controlling the applied forces during sharp probe interaction with the samples. We have purchased commercial piezoresistive cantilever based dynomometer probes and are planning to build more sensitive ones. Attaching this kind of piezoresistive or mechanical probes to a piezoelectric inertial nanomanipulator allows measurement of the stiffness and compliance of a wide range of materials. If the interaction is made under electron microscopy observation, we can easily gain a practical experience on the interaction of sharp tips with materials. We show here how a commercial AFM probe can be stressed and used to dig pits on a gold surface (movie), how we can measure the sensitivity of insects’ tactile hair sensors (movie), how we can measure the compliance of carbon nanotubes wool (movie), and even how we can accomplish a micrometric electrical switch based on a carbon nanotubes helical spring (movie).
UHV STM with microscopic spectral analysis capabilities
This facility based at the ENEA laboratories is hosted in a 35 l ionically pumped ultra-high vacuum vessel endowed with a home made STM, a Z inertial piezo slider for sample approach and a remotely controlled ρ,θ,φ nanomanipulator for sample movement with submicron precision. The vessel has two load-lock chambers for tip and sample load and a hand actuated wobbling pincer for manual operations. It also has optical ports for access to the tip region by two different laser beams.
Above on the left is the scheme of the UHV STM system coupled to the available laser sources and to the spectroscopic analysis system devoted to “tip-enhanced” photoluminescence studies. On the right, a photograph of the UHV vacuum vessel with its lid lifted shows details of the optical microscope case and of the STM. Below, an optical microscope image of the STM tip over a layer of Rhodamine 6G fluorescent dye in white light (left) and excited by a green laser (540 nm) as recorded through a long wavelength pass filter with cutoff at 560 nm with the tip within tunnelling distance (center) and 5 μm away from the surface (right). Light from the tip-enhancement area is spatially selected by a moveable pinhole and analyzed by a monochromator producing the R6G emission spectrum reported below.
Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectrometry (EDS)
LEO 35 Field Emission by Zeiss©
Characteristics
• GEMINI® based Field emission e-beam column capable of ultra high resolution SEM up to 25 KV.
• Achieves a similar low energy spread compared to a cold field emission source, but with a much higher emission current and much higher beam stability.
• Featuring a TV camera and three detectors, i.e. (i) secondary electrons detector, (ii) back-scatter electrons detector (EBSD) , (iii) In-lens detector for ultra high resolution imaging (<10nm).
• Equipped with an energy dispersive x-Ray spectrometer (EDS) from Oxford INCA© allowing for acquisition of sample emitted x-rays. Accompanying software with embedded database of reference spectra for elements identification/recognition, compositional nano-analysis and x-ray mapping.
• Automated stage controlled by a joystick with x-y-z translations, z rotation (full 360° range) and x tilting (up to 90° tilt) for ultra high precision sample positioning.
• Vacuum system (a rotary pump plus a turbo pump) to achieve chamber vacuum of less than 2 x 10-6 mbar.
• Dynamic vibration-damper.
• Several stubs settings and adaptors for mounting samples on the stage.
• Sample preparation desk.
• Sputter-coater to metallize non-conductive samples (with gold or other target).
Some Results
SEM is a common tool to address a variety of aspects in material research and nanotechnology. For illustrative purposes, some examples of SEM work in a variety of research areas (e.g energy, environment, biotechnology and nanotechnology in general) are reported here. As far as energy and environment are concerned, ceramics and nanostructured materials are key in applications such as fuel cells and chemical sensors. Fig.1 shows a surface micrograph of a composite porous electrode (cathode) designed for Solid Oxide Fuel Cell applications and fabricated by Pulsed Laser Deposition technique. Pd-Au metal droplets have been deposited and condensed on a continuous open La0.8Sr0.2Co0.8Fe0.2O3-δ network, previously grown on a dense polycrystalline stabilized ZrO2. Mixed ionic and electronic properties of La0.8Sr0.2Co0.8Fe0.2O3-δ assisted by the electrical properties of metal droplets at high temperature, reduce the cathode polarization in SOFCs.
Figure 1. Surface of a composite porous electrode (cathode) designed for SOFC applications and fabricated by Pulsed Laser Deposition technique. Pd-Au metal droplets were deposited onto a continuous open La0.8Sr0.2Co0.8Fe0.2O3-δ network grown on dense polycrystalline stabilized ZrO2.
Another class of fuel cells are based on the usage of polymeric “Proton Exchange Membranes” (PEM) as the fuel cell electrolyte. Nafion is the best performing polymer currently available for this application but, amongst other drawbacks, suffers of a marked drop in conductivity for working temperatures above 90°C due to morphological transitional and de-hydration. A variety of composite membrane, i.e. hybrid structures of Nafion with dispersed fillers, are under investigation to overcome this problem. Figure 2 shows an example of Nafion mixed with an inorganic phase “sulfonate diphenyl silane diol” (SDPSD), appearing as “bright” islands in the photos. The PEM performances improve based on the content and distribution of the SDPSD. Thermal treatment is one way of controlling the microstructure of the composite structure. SEM micrographs clearly shows that heating the samples at 170 °C induces a finer dispersion of the filler, yielding more minute SDPSD clusters. The limited electronic conductivity of this material makes it difficult to analyze with the SEM.
Figure 2. Two SEM pictures of the Nafion/SDPSD 70:30 composite membrane for PEM fuel cells. Photos selected demonstrate the effect of the thermal treatment at 170°C on the dispersion SDPSD in the microstructure. Sample “as-prepared” (far left) and the “heat-treated” one (left) exhibit different dispersion of SDPSD islands within the Nafion matrix, highlighting a size reduction of the SDPSD clusters induced by the thermal treatment.
SEM imaging is also crucial for the characterization of nanostructured materials for biological applications. Within biotechnology, tissue engineering combines the fields of engineering, chemistry, biology, and medicine to fabricate replacement tissues able to restore, maintain, or improve structurally and functionally damaged organs. Polymeric scaffolds for stem cells growth and differentiation represent an active research topic. Some scaffolds are fabricated by electrospinning as highly interconnected porous polymeric networks. Figure 3 shows a SEM image of nanostructured PLA electrospun nanofibers endowed with nanopores. Such fibres are multiscale features, having lengths in the centimetre range, micrometric and sub-micrometric diameters and nanoscale roughness/porosity. The various length-scales can be largely tuned and tailored through processing parameters.
Figure 3. SEM image of PLA electrospun nanofibers with nanopores. The fibres have a multiscale geometry with lengths in the centimetre range, diameters between few tens of nanometers and few micrometers, and nanoscale porosity. Length-scales can be tuned and tailored through processing parameters. Controlled porosity is important also for drug delivery.
The topics briefly highlighted in the examples are discussed in greater details in several papers. The interested reader can consult the selected papers cited in the reference.
SEM capabilities go beyond pure geometrical and morphological information, highlighted in the previous examples. In fact backscattered electrons and EDS can be used to obtain compositional data. In the next example in Figure 4, a carbon nanoparticle of about 400 nm deposited from the environment onto a substrate made of nanoporous Si. The X-ray spectra shown on the left were collected with the EDS at two different locations pointed out in the SEM micrograph, i.e. at point (1) centered on the nanoparticle and at point (2) away from it. From comparison, the missing peak in the spectrum collected at site (2) is characteristic of C, which reveals the chemical nature of the nanoparticle.
Figure 4: Micro-analisys by EDS of a 400nm carbon particle (site 1) deposited from the environment onto a nanoporous Si substrate. The nature of the nanoparticle is highlighted as a “difference”, because the yellow spectrum at site (2) is missing a peak characteristic of carbon and present in the red spectrum at site (1).
References on SOFC and PEM
[1] D. Z. de Florio, R. Muccillo, V. Esposito, E. Di Bartolomeo and E. Traversa, “Preparation and Electrochemical Characterization of Perovskite/YSZ Ceramic Films”, J. Electrochem. Soc., 152 (1), A88, (2005).
[2] V. Esposito, D. Z. de Florio, F.C. Fonseca, E.N.S. Muccillo, R. Muccillo and E. Traversa, “Electrical properties of YSZ/NiO composites prepared by a liquid mixture technique”, J. Eur. Ceram. Soc., 25, 2637, (2005).
[3] Deganello, V. Esposito, E. Traversa and M. Miyayama. “Cathode performance of nostructured La1-aSraCo1-bFebO3-d on a Ce0.8Sm0.2O2 electrolyte prepared by citrate-nitrate auto-combustion”, F J. Electrochem. Soc., 154 (2), A89, (2007).
[4] Barbara Mecheri, Alessandra D’Epifanio, Enrico Traversa, Silvia Licoccia. “Effect of an ormosil-based filler on the physico-chemical and electrochemical properties of Nafion membranes”, J. Power Sources, 169 (2007) 247–252.
References on Chemical Sensors
[5] M.L. Grilli, E. Di Bartolomeo, A. Lunardi, L. Chevallier, S. Cordiner and E. Traversa, “Planar Non-Nernstian electrochemical sensors: field test in the exhaust of a spark ignition engine”. Sensors and Actuators B, 108 (2005) 319-325
[6] L. Chevallier, M.L. Grilli, E. Di Bartolomeo and E. Traversa. “Non-Nernstian planar sensors based on YSZ with Ta (10 at.%)-doped nanosized titania as sensing electrode for high temperature applications”. International Journal of Applied Ceramic Technology, 3 [5] 393-400 (2006)
References on Tissue Engineering
[7] E. Traversa, B. Mecheri, C. Mandoli, S. Soliman, A. Rinaldi, S. Licoccia, G. Forte, F. Pagliari, S. Pagliari, F. Carotenuto, M. Minieri, P. Di Nardo. “Tuning Hierarchical Architecture of 3D Polymeric Scaffolds for Cardiac Tissue Engineering”, J. Exp. Nanoscience, (accepted sept. 2007).
Atomic Force Microscope (AFM) & Scanning Tunnelling Microscopy (STM)
The Variable Temperature STM/AFM
Characteristics
Stainless steel UHV system (Standard pressure 6 10-11mbar).Turbo molecular pump for roughing: 240 l/sec – Ionic pump: 500 l/sec – Titanium sublimator.X-Y-Z manipulator with direct current or resistive heating. Tmax=1500 K.STM/AFM (piezoresistive).Heating at the STM position: up to 1500 K.Cooling at the STM position: down to 25 K.E-beam evaporators at the STM position: Ge and Si.Reverse view LEED-Auger with LaB6 filament.
Some Results
Ge/Si(111) – experiments performed were related to the visualization of the growth by Physical Vapour Deposition of Ge nanostructures on 7×7 Si(111) reconstructed surfaces. By evaporating Ge on Si(111) at T=500°C the formation ofwetting layers have been studied by Scanning Tunneling Microscopy in situ. The evolution of the Ge islands appearing after the wetting layer (completed at 3 ML). The 3D islands appear as truncated tetrahedrons (7×7 reconstructed on the top) and evolves into rounded shape, flat islands with a central hole. An erosion of the substrate around the islands has been also evidenced and measured. The island have lateral dimensions in the range 200 – 500 nm. The statistical distribution of the islands shapes and contact angles has been analysed. On Si step bunched surfaces (obtained by flashing at 1200°C in proper condition the Si substrate) the self aggregation and ordering of the islands has been evidenced. These results have been published in several papers and conference or school proceedings [1-6].
Fig 1. Ge islands grown on Si(111) observed by STM and AFM at various stages of evolution. Top left, a) an island after the nucleation (STM: 236x236x8 nm), top rigth, b) new facets (100 and 117) appear (STM: 230x230x38 nm); bottom left, c). First stage of ripening. (STM: 527x527x12 nm) bottom rigth, d) final stage of ripening (AFM image: 527x527x10 nm). Ge flux was 1 Å/min. The substrate temperature was 530 °C for a), 450 °C for b) and 500 °C for c) and d).
Fig 2. Organization of the Ge islands on a Si(111) step-bunched substrate a) STM image of 2.5 nm Ge deposition on Si(111) at T=450 °C 2.7×3.7 μm2. The total height of the image is 56 nm. b) STM image of 6 nm Ge deposition on Si(111) at T=450 °C 10×10 μm2. The total height of the image is 82 nm.InAs/GaAs quantum dotsThe InAs quantum dots on GaAs were prepared ex-situ by MBE as follows: the GaAs(001) wafer was initially deoxidized in As flux at 640 °C until a weak 2×4 RHEED pattern appeared. Afterwards, the substrate temperature was lowered to 590 °C and an epitaxial GaAs buffer layer of approximately 0.75 mm was grown at a rate of 1 μm/h. After 10 min of annealing, the temperature was further lowered to 500 °C for the InAs growth. The deposition rate was 0.028 ML/s with an In/As flux ratio of 1/15. The InAs coverage was 3 ML (the 2D-3D transition occurs at 1.6 ML). A 50 nm As capping layer is grown on top of the quantum dots, in order to protect the surface during the transfer to the AFM/STM chamber if atomic resolution and reconstruction have to be obtained by STM. The other samples are grown on intrinsic GaAs exhibiting a resistance too high to be imaged by STM, so the measurements are normally performed by AFM. Many samples, with different InAs coverages and growth modes have been prepared and measured. The size and distribution of the dots obtained in various growth conditions have been measured. Two main growth modes have been analysed: Continuous (the In flux was never interrupted) and Migration Enhanced (the In flux was interrupted at regular intervals, in order to increase the atomic movements ad aggregation on the surface). The island size increases in the Migration Enhanced mode. The onset of the quantum dot formation has been evidenced by also by STM, by imaging 2D islands after which act as precursors. The results have been published in two papers [7-8]
Fig 3. STM images at 1.3 ML of InAs on GaAs(001); a) 2D-islands of average height 0.4 nm (precursors); b) high resolution image of the wetting layer. The RHEED image has been acquired in the MBE system at the end of the growth.
Fig 4. Needle-Sensor AFM of self-assembled quantum dots of InAs on GaAs(001) imaged using the VT SPM; dimensions: 300×300 nm. Height: 10 nm. The typical dot size is around 25 nm, and the height is 7 nm.
References on Ge/Si
[1] F. Boscherini, G. Capellini, L. Di Gaspare, F. Rosei, N. Motta and S. Mobilio, “Ge-Si intermixing in Ge quantum dots on Si(001) and Si(111)”, Appl. Phys. Lett. 76, 682 (2000).[2] F. Rosei, N. Motta, A. Sgarlata, G. Capellini and F. Boscherini, “Formation of the Wetting Layer in Ge/Si(111) studied by STM and XAFS”, Thin Solid Films 369 p29 (2000).[3] F. Boscherini, G. Capellini, L. Di Gaspare, F. Rosei, N. Motta and S. Mobilio, “Atomic intermixing in Ge Quantum Dots”,“ESRF Highlights” 1999, Surfaces and Interfaces (may be visualized on the web site www.esrf.fr).[4] F. Boscherini, G. Capellini, L. Di Gaspare, M. De Seta, F. Rosei, N. Motta A. Sgarlata and S. Mobilio, “Ge-Si intermixing in Ge quantum dots on Si”, Thin Solid Films 380, 173 (2000).[5] A. Sgarlata, F. Rosei, M. Fanfoni, N. Motta and A. Balzarotti, “STM/AFM study of Ge Quantum Dots grown on Si(111)”, IEEE Proceedings of the 11th International Semiconducting and Insulating Materials Conference, February 2000.[6] F. Rosei, N. Motta, A. Sgarlata and A. Balzarotti, “Growth and characterization of Ge nanostructures on Si(111)”, submitted for publication to Lecture Notes in Physics. (February 2001).[7] Nunzio Motta Self-assembled Quantum Dots studied by scanning probes and other structural techniques. Proc. Workshop on Nanotubes and Nanostructures 2000, S.M.Pula (CA), Ed. S.Bellucci, (Editrice Compositori 2001)[8] N. Motta, F. Rosei, A. Sgarlata, G. Capellini, S. Mobilio, F. Boscherini, Evolution of the intermixing process in Ge/Si(111) Self-assembled islands, submitted to Materials Science B
References on InAs/GaAs
[1] F. Patella, M. Fanfoni, F. Arciprete, S. Nufris, E. Placidi, and A. Balzarotti; Kinetic aspects of the morphology of self-assembled InAs quantum dots on GaAs(001) Applied Physics Letters 76 (2001) 320.[2] Arciprete, A. Balzarotti, M. Fanfoni, N. Motta, F. Patella, A. Sgarlata; Morphology of self-assembled quantum dots of InAs on GaAs(001) and Ge on Si(111) in Recent Research developments in Vacuum Science & Technology. Publisher: Transworld Research Network (2001).
Molecular Beam Epitaxy & Electron Spectroscopy
Molecular Beam Epitaxy
Characteristics
(Mod.32; Riber)Stainless steel UHV system (Standard pressure 6 10-11 mbar). Turbo molecular pump for roughing: 240 l/sec – Ionic pump: 500 l/sec – Titanium sublimator. X-Y-Z manipulator with resistive heating.
Tmax=800 °C. Knudsen cells: Ga, As, In, Al, Si (as dopant). Rheed optics E= 10 KeV.
High Resolution Electron Energy Loss Spectroscopy
HREELS cross section
Characteristics
Stainless steel UHV system (Standard pressure 4×10-11mbar). Turbo molecular pump for roughing: 240 l/sec – Ionic pump: 200 l/sec – Titanium sublimator. X-Y-Z manipulator with resistive heating. T max=800 °C. Leed optics.
New neutron facility for fast neutron irradiation tests of electronics
Collaborative research among Italian and British scientists and engineers from Nast Centre for Nanoscience Nanotechnology and Innovative Instrumentation, CNR (I), ISIS Spallation neutron Source (UK), Universities of Central Lancashire, Padova and Milano Bicocca, and from the avionics and aerospace industries have been using VESUVIO neutron flux…