Scanning Probe Microscopy

1. Growth dynamics-kinetic roughening of nanocluster films

Recently a strong impetus is given to studies of nano-cluster/nano-structured thin layers for two main reasons. The first stems from the desire to miniaturise electronic devices. Specifically, one would like to grow organised nanometer-size islands with specific electronic properties. The second subfield of nano-structured materials, is thin or thick films which show mechanical and magnetic properties different from their microcrystalline counterparts. The precise reasons for these effects is currently being investigated, but one can cite the presence of a significant fraction of atoms in configurations different from the bulk configuration, for example, in interfaces. Interestingly the clusters are grown in extreme non-equilibrium conditions, which allow one to obtain metastable structures or alloys. Because one avoids the effects of nucleation and growth on a specific substrate with this method one may tune the properties of the films by choosing the appropriate preparation conditions [1]. In addition, nanocluster films are also of strong interest to catalysis studies due to large surface to volume ratio owing to the porous structure of films build by soft landing nanoclusters [1].  At any rate the formation of ordered aggregates of nanoparticles exhibit interesting magnetic, electronic and/or opto-electronic properties. At the present nanocluster source (obtained from Oxford Applied Research) metal atoms are generated by magnetron discharge and clustered by the so called gas aggregation method [1]. Briefly metal atoms due to collisions with Ar atoms loose energy and combine to form clusters which are subsequently jet-propelled through a nozzle to form a cluster beam. 

• Our present studies include Cu, Fe and Nb nanocluster films [2, 3]. Their study is performed by means of Atomic force microscopy (AFM), magnetic force microscopy (MFM), and high resolution transmmision electron microscopy (HRTEM).

• Growth of Cu nanocluster films [2]: Up to now, growth front aspects of Cu nanocluster films deposited with low energy (soft landing films) onto silicon substrates at room temperature have been investigated by AFM [6]. Analyses of the height-difference correlation function yield a roughness exponent H=0.45± 0.05. The rms roughness amplitude w evolves with deposition time as a power law with exponent b =0.62± 0.07, leading also to a power law increase of the local surface slope r  with an exponent c=0.73± 0.09. The measured scaling exponents, in combination with an asymmetrical height distribution, point at a complex nonlinear roughening mechanism dominated by the formation of pores.

         Cu nanocluster films (open and closed films). Rms amplitude w vs. deposition time

• TEM studies of Fe nanoclusters [3]: Further, HRTEM has been employed to examine the structural stability of nano-sized Fe clusters as deposited and after in-situ annealing treatments. The thin oxygen shell that is formed around the Fe clusters (upon air exposure) is of the order of 2 nm surrounding a 5 nm core of bcc iron.  The oxygen shell breaks down upon annealing at relatively low temperatures (~ 500° C) leading to pure Fe particles having a bcc crystal structure. The truncated rhombic dodecahedron was found as the possible shape of the clusters, which differs from former theoretical predictions based on calculations of stable structural forms.

        AFM images of Fe nanoclusters                                                    TEM images of Fe nanoclusters (left: oxide shell structure)

Related references

[1] H. Haberland M. mosseler, Y. Qiang, O. Rattude, T. Reiners, and Y. Thunrner, Surf. Review and Lett. 3, 887 (1996); H. Haberland, M. Mall, M. Moseler, Y. Quiang, Th. Reiners, and Y. thurner, Nucl. Instrum. Methods Phys. Res. Sect. B 80/81, 1320 (1993); G. Fuchs, P. Melinon, F. Santos Aires, M. Treileux, B. Cabaud, and A. Hoareau, Phys. Rev. B 44, 3926 (1991); C. G. Zimmermann, M. Yeadon, K. Nordlund, J. M. Gibson, R. S. Averback, U. Herr, and K. Samwer, Phys. Rev. Lett. 83, 1163 (1999).

[2] G. Palasantzas, S. A. Koch, J. Th. M. De Hosson, "Growth front roughening of room temperature deposited copper nanocluster films" .Appl. Phys. Lett. 81, 1089 (2002)

[3] T. Vystavel, G. Palasantzas, S. A. Koch, J. Th. M. De Hosson, Nano-sized iron clusters investigated with in-situ transmission electron microscopy, Appl. Phys. Lett. 82, 197 (2003)

2. Scanning Auger/Electron microscopy 

We have employed UHV-SAM/SEM to study grain boundary segregation in combination with in-situ specimen fracture. Systems studied include Al-Mg, Cu-Sb, Cu-Bi, Ni-Al alloys. 

  Fracture of Cu matrix with Bi precipitates                              Fracture of a Cu matrix with Sb (SAM map of Sb)

Furthermore, significant focus has been given to electron beam induced oxidation of specimen surfaces such as polycrystalline Ni-Al and Al-Mg alloy surfaces. Indedd, electron beam currents of a few nano-amperes, currently used in nanometer scale scanning Auger/scanning electron microscopy, promote oxidation of polycrystalline Ni3Al  to a degree that depends on the size of the beam.

Oxidation of Ni3Al with variable beam size (SAM map of O)

In fact, at small beam sizes (£ 10 m m) the oxidation of Ni3Al at room temperature follows a model based on the premise that the electron beam creates additional nucleation sites around which oxide growth occurs. With increasing beam size the oxidation process becomes slower and O chemisorption plays a significant role. As a result the Ni-oxide depth decreases drastically with an increasing spot size. It offers an alternative way to monitor the NiO thickness in the nanometer range. 

Finally, scanning auger analysis has been performed for W macroscopic contacts onto Si  for conduction measurements through nanodevices fabricated within the gap shown in the SEM image below.

    SEM  for nanocontacts              SAM-W                                 SAM-Si

References

[1] D. T. L. van Agterveld, G. Palasantzas and J. Th. M. De Hosson, "Effects of Cu-Sulphide precipitates in Cu upon impact fracture: An ultra-high-vacuum study with Scanning Auger/Electron Microscopy" Act. Mater. 48, 1995 (2000).

[2] S. A. Koch, D. T. L. van Agterveld, G. Palasantzas and J. Th. M. De Hosson, "B segregation on grain boundary surfaces of intergranular fractured Ni-Al-alloys under UHV conditions" Surf. Sci. 482-485, 254 (2001).

[3] S. A. Koch, D. T. L. van Agterveld, G. Palasantzas and J. Th. M. De Hosson, "Electron beam induced oxidation of surfaces of Ni₃Al- base alloys" Surf. Sci. Lett. L67, 476 (2001).

[4] G. Palasantzas, D. T. L van Agterveld, S. A. Koch, J. Th. M. De Hosson , Flux effects on electron beam induced oxidation of Ni₃Al surfaces, J. Vac. Sci. Technol B 18, 2472 (2001).

3. Growth dynamics-kinetic roughening of organic thin films

During recent years there has been an increasing technological interest in organic thin films, either polymers or oligomers, as the active layer in molecular devices such as light-emitting-diodes, photovoltaic devices and field-effect-transistors. Injection, transport and recombination of charge carriers depend among other parameters on molecular packing, grain boundaries and roughness along the formed interfaces. As a result, control of the film morphology is of primary concern for the optimization of electro-optical properties of organic-based photonic devices. The former studies of oligomer and polymer films have clearly pointed out the importance of structural organization at higher levels [1].

Ooct-OPPV5 film at RT and 100 °C substrate temperature (right images)

Up to now we have investigated growth front scaling aspects for PPV [pentamer 2,5-di-n-octyloxy-1,4-bis(4-(styryl)styryl)-benzene] oligomer thin films vapour-deposited onto silicon substrates at room and elevated temperature [2, 3]. The measured roughness exponents H increase from H» 0.4 at low substrate temperatures where growth is dominated by vacancy formation, to H» 0.7-0.8 at elevated temperatures where diffusive growth takes place. Moreover, the rms roughness amplitude and the correlation length evolve with temperature closely as an Arrhenius process with activation barrier comparable to molecule transnational and rotational barriers on oligomer surfaces. Furthermore, studies are planned to relate surface disorder (and thus associated scaling exponents) and more in general structure to electrical transport properties for these oligomer thin films. The ultimate challenge is to build molecular nanoscale devices by getting insight into fundamental principles that relate molecular structure and corresponding opto-electronic processes.

Related references

[1] Polymers for Electronic and Photonic Applications, edited by C.P. Wong (Academic Press, Boston, 1993); T.-M. Lu and J. A. Moore, Mat. Res. Soc. Bull. 20 (1997) 28; F. Biscarini, P. Samorí, O. Greco, and R. Zamboni, Phys. Rev. Lett. 78 (1997) 2389; F. Biscarini, R. Zamboni, P. Samori, P. Ostoja, C. Taliani, Phys. Rev. B 52 (1995) 14868; Y.-P. Zhao, J. B. Fortin, G. Bonvallet, G.-C. Wang and T.-M. Lu, Phys. Rev. Lett. 85 (2000) 3229; H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A. J. Spiering, R. A. J. Janssen, P. Herwig, D. M. de Leeuw, Nature 401 (1999) 685.

[2] D. Tsamouras, G. Palasantzas, J. Th. M. De Hosson, Growth front roughening of room temperature vapor deposited ppv-oligomer films, Appl. Phys. Lett. 79, 1801 (2001); D. Tsamouras, G. Palasantzas, J. Th. M. De Hosson, "Roughening aspects of room temperature vapor deposited oligomer thin films onto Si substrates". Surf. Sci. 507-510, 357 (2002)

[3] D. Tsamouras and G. Palasantzas, "Temperature dependence of the growth front roughening of oligomer films". Appl. Phys. Lett. 80, 4528 (2002).

4. SPM based nanofabrication/nanotechnology

The advancement of scanning probe microscopy (SPM) techniques allowed the miniaturisation down to nanoscales of electronic devices opening thus the possibility for further exploitation of quantum mechanical phenomena at room temperature. Indeed, in the recent years there has been an outstanding development of new methods for micro- and nanofabrication techniques in terms of SPM which will be essential to scientific progress in many areas in physics, materials science, Chemistry, and biology. They will form enabling technologies for applications ranging nanoelectronics to molecular electronics to micro-optical components to nanoelectromechanical systems to catalysis. Advances are strongly aided by the highly engineered and successful lithography techniques that are used in microelectronics. One of the fundamental limits in lithography is imposed by the properties of the resist layer since for the smallest feature size one would like to the thinnest resist [1]. 

STM tip induced H depassivation                   Co-silicide  nanowires of width 4-5 nm                

In the near past it was used UHV-STM tip based fabrication of nanoscale devices on H-passivated Si(100) surfaces. Combination of STM based electron stimulated H desorption, and subsequent metal deposition of (i.e., Al, Fe, Co, Pd et.) resulted in nanowires with width down to 4-10 nm [1, 2]. Such a selective nanofabrication scheme can be also applied to fabricate molecular besides metal based nanodevices. 

Related references

[1] G. Palasantzas, J. Th. M. De Hosson, and L. J. Geerligs, "UHV-SPM Nanofabrication", Invited (By Dr. Nalwa) Accepted for the Encyclopedia in Nanotechnology (2003).

[2] G. Palasantzas, B. Ilge, J. M. M. de Nijs, and L. J. Geelings, J. Appl. Phys. 85, 1907 (1999); B. Ilge, G. Palasantzas, and L. J. Geelings, Appl. Surf. Sci. 144-145, 543 (1999); G. Palasantzas, B. Ilge, S. Rogge, and L. J. Geerligs, Microelectronics Engineering 46, 133 (1999).

5. Nanostructured Biomaterials

Nanostructured biomaterials offer outstanding optical and wetting ability properties. Specific attention is paid to the corneal nipple array of moth eyes, which has inspired technical applications in the area of antireflective coatings [1] and self-cleaning of superhydrophobic structures [2]. Related structures are found on the fragile, transparent wings of several insect species and on the leaves of certain plants, notably the lotus. Moreover, butterfly wings often replicate photonic band-gap materials [3].

Nanostructured nipple arrays of  butterfly eyes (AFM image)

Related references

[1] S. J. wilson and M. C. Hutley, Optica Atca 29, 993 (1982); P. B. Clapham, and M. C. Hutley, Nature 244, 281 (1973).

[2] W. Barthlott and Neinhuis, Planta 1-8, 202 (1997); W. Barthlott, Scanning electron microscopy of the epidermal surface in plants. In: D. Claugher (ed) Scanning electron microscopy in taxonomy and functional morphology. Clarendon Press, Oxford, pp. 69-94 (1990); W. Barthlott (1993) Epicuticular wax ultrastructure and systematics. In: Behnke HD, Mabry TJ (eds) Evolution and systematics of the Caryophyllales. Springer, Berlin, pp. 75-86.

[3] L. P. Biro, Zs. Balint, K. Kertesz, Z. Vertesy, G. I. Mark, Z. E. Horvath, J. Balazs, D. Mehn,3 I. Kiricsi, V. Lousse, and J.-P. Vigneron, Phys. Rev. E 67, 021907 (2003).