Università Cattolica del Sacro Cuore

Surface Science and Spectroscopy

Fig.1 - Surface Science and SpectroscopyDiluted magnetic semiconductors and oxides

Diluted magnetic semiconductors (DMS) and oxides (DMO) are the materials at the basis of novel devices for spintronics applications. Our research is focussed on the interplay between electronic properties and magnetism in bulk crystals, surface alloys, and multilayers. Most of experiments are based on electronic spectroscopies with synchrotron radiation sources, mainly at the BACH and ALOISA beamlines of the Elettra Synchrotron in Trieste.
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Fig.2 - Surface Science and SpectroscopyGrowth, electronic structure, and functional properties of eumelanin thin films and related interfaces

Eumelanin has recently attracted the attention of research groups involved in the study of functional organic materials, with the goal to identify proper bio-organic materials for applications in the optic, electronic and catalysis fields. Only recently, by using soft X-ray spectroscopies for the analysis of eumelanin aggregates, we have been able to measure for the first time the density of states of both occupied and unoccupied electronic levels and we have shown to which extent the calculated electronic structure of single monomers catches the main features of solid state aggregates.  Read more...

Fig.3 - Surface Science and SpectroscopyFunctional properties of nanostructured interfaces and architectures

Nanostructured interfaces and architectures are proving novel schemes and functionalities for a large number of devices, including solar cells, chemresistor gas sensors, field effect emitters. Our lab is involved in a number of collaborations aimed to study the interaction on nanostructured carbon systems with molecules of both oxidizing and reducing gases.  Read more...

Fig.4 - Surface Science and SpectroscopyOxide heterointerfaces

When a LAO-STO heterointerface is created, an n-type heterostructure is obtained with a TiO2 plane (electron doping) termination of the STO substrate. In this case, the LAO-STO interface becomes conducting and yields a quasi-2D electron gas (2DEG). The transition to the metallic state is known to be thickness dependent: the 2DEG is observed only when capping is at least 4 u.c. (unit cell) thick. Our lab is involved in tracking the 3d1 electronic states involved in the formation of the 2DEG.  Read more...


Diluted magnetic semiconductors and oxides

Across the years we have investigated several systems such as (i) the Mn-Ge alloy, (ii) Mn-doped CdTe single crystal and surface alloys, (iii) Mn-doped GaSe and (iv) several rutile based diluted magnetic oxides. For the magnetic properties, our research was supported by MC Mozatti and P. Galinetto (University of Pavia), while for the thin film and single crystal growth, the collaboration was extended to L. Malavasi (Univ. of Pavia), V. Aguekian (University of St. Petersburg) and G. Karczewski (Polish Academy of Sciences).

(i) Since the discovery of ferromagnetism in the MnxGe1-x DMS [1], the Mn-Ge system as played a central role for possible applications in the field of spintronics. This role has been recently tested with devices based on Mn0.05Ge0.95 quantum dots [2]. Indeed, the Mn-Ge system has shown to be a very rich playground for testing spintronics architectures, ranging from the MnxGe1-x diluted alloy (x=0.01 to 0.05) [1], to the MnGe2 nanocolumns [3], and from a number of Mn:Ge(100) or Mn:Ge(111) interfaces [4, 5] to amorphous, ferromagnetic, Mn-Ge thin films [6].

[1] Y. D. Park, et al., Science, 295 (2002) 651
[2] F. Xiu, et al., ACS Nano, 4 (2010) 4948
[3] M. Jamet, et al., Nature Materials, 5 (2006) 653
[4] C. Zeng, et al., Appl. Phys. Lett., 83 (2006) 5002
[5] L. Sangaletti, et al., Phys. Rev. B, 72 (2005) 35434
[6] S. Guchhait, et al., Phys. Rev. B, 84 (2011) 024432

Our main interest is the epitaxial Mn:Ge(111) interface the early stages of growth. This interface is largely unexplored and a combination of growth temperature, layer thickness and post growth treatments can provide a rich scenario determined by the interplay of magnetism and electronic properties. The aim of our activity is to draw the phase diagram of this system by combining state-of-the-art electron spectroscopies, including band mapping with angle-resolved photoemission spectroscopy and magnetic circular dichroism with X-rays.

(ii) Mn-doped cadmium telluride is a well-known diluted magnetic semiconductor, which displays interesting magnetic properties, ranging from paramagnetism to antiferromagnetism and spin-glass behaviour. With respect to ferromagnetic (Ga,Mn)As, that shows a relatively high TC and the possibility to achieve a suitable doping, (Cd,Mn)Te has traditionally been difficult to dope (e.g., nitrogen doping), and only recently claims of ferromagnetism in (Cd,Mn)Te have
been reported, which make this DMS class of materials apparently less suitable for applications. However, along with other II-VI DMS, the interest on (Cd,Mn)Te is still very high. With respect to (Ga,Mn)As, these materials and their heterostructures are relatively easy to prepare both in bulk
and in thin layers. This opens the possibility to use these materials for testing new concepts for spintronic applications based in particular on low-dimensional structures such as quantum wells QWs, quantum dots, or digital alloys.
Our main achievements in this field are the study of electronic states of Mn in buried (Cd,Mn)Te layers within a CdTe-(Cd,Mn)Te superlattice [7], and the study, through resonant photoelectron diffraction (ResPED), of the local electronic properties of Mn cations during the build-up of the Mn:CdTe interface [8]

[7] L. Sangaletti, et al., Appl. Phys. Lett., 96 (2010) 142105
[8] L. Sangaletti, et al., Physical Review B, 81 (2010) 245320

(iii) One of the most intriguing properties related with DMO is the evidence of magnetism in formally d0 systems (i.e. with closed shell cations). Once the effects of sample contamination on magnetism are excluded, the origin of ferromagnetism and the underlying mechanism remain elusive and strongly debated. The research activity on the electronic properties of diluted magnetic oxides [9], is focussed on the TiO2-rutile host system. This activity involves the growth by r.f.sputtering of thin films doped with 3d transition metals (Cr to Cu) , N and C, and the experimental study of their electronic and magnetic properties [10-13]. Ab-initio calculations of the electronic structure of doped-TiO2, accounting for the effects of dopants, defects, and oxygen vacancies is becoming a standard tool in our activity, and represents one of expanding fields of our investigations.

[9] M. Venkatesan, et al., Nature, 430 (2004) 630
[10] L. Sangaletti, et al., J. Phys. Cond. Matter, 18 (2006) 7643
[11] L. Sangaletti, et al., Physical Review B, 78 (2008) 075210
[12] L. Sangaletti, et al., Physical Review B, 80 (2009) 033201
[13] G. Drera et al., Applied Physics Letters, 97 (2010) 012506

(iv) Our most recent investigations on DMS are those carried out on Mn-doped GaSe. The III-VI semiconductors GaSe, InSe, GaTe, and GaS have received considerable interest in the last few
years because they show remarkable nonlinear optical properties and they are regarded as promising materials for photo-electronic applications, even in the form of nanowires. In the particular case of GaSe and GaS, the interest on these systems has been recently renewed due to the possibility to obtain ultrathin layer transistors based on atomic-thin sheets.
The magnetic properties of these systems doped with transition metal ions (e.g. Mn, or Fe-doped GaSe) are also under investigation with the aim to find out new classes of diluted magnetic semiconductors (DMS) of the form AIII1-xMxBVI , where AIIIBV I is a III-VI semiconductor and M is a transition metal ion. We have carried out a thorough study of the electronic properties of Mn-doped GaSe on the basis of resonant photoemission spectroscopies [14].

[14] S. Dash, et al., arXiv:1303.1984, 8th March 2008


Growth, electronic structure, and functional properties of eumelanin thin films and related interfaces

Eumelanin has recently attracted the attention of research groups involved in the study of functional organic materials, with the goal to identify proper bio-organic materials for applications in the optic, electronic and catalysis fields [15]. For example, its strong absorbance and semiconducting characteristics render melanin a possible candidate for application in dye sensitised (organic) solar cells. Despite extensive experimental and theoretical studies conducted on both natural and synthetic melanins, the structure, composition and aggregation behaviour of this class of pigments remain unknown. This is part due to the fact that melanins are difficult molecules to study, because they strongly polymerize in disordered aggregates and are virtually insoluble in the most common solvents. Only recently, by using soft X-ray spectroscopies for the analysis of eumelanin aggregates, we have been able to measure for the first time the density of states of both occupied and unoccupied electronic levels and we have shown to which extent the calculated electronic structure of single monomers catches the main features of solid state aggregates [16, 17]. The research activity is being carried out in collaboration with Andrea Goldoni (Elettra, Trieste) and with the ALOISA beamline staff at ELETTRA (IOM-CNR, Trieste), as well as with the support of theoretical calculations of Ralph Gebauer at ICTP and Prasenjit Ghosh.. We have also devised a route for doping with alkali metals, namely K incorporation in thin melanin films during the growth in an electrochemical cell [18].
Important similarities are found with porphyrins, as one of the proposed models for the eumelain structure is a cyclic macromolecule with 4 N atoms inside the cycle, a structural feature shared with porphyrins. With the use of extensive simulation of core level spectroscopies we have validated the macrocyclic model of eumelanin, addressing the most likely set of the four monomers contributing to the macrocyclic molecule [19].

[15] J. P. Bothma, et al., Advanced Materials, 20 (2008) 3539
[16] L. Sangaletti, et al., J. Phys. Chem. B, 111 (2007) 5372
[17] L. Sangaletti, et al., Physical Review B, 80 (2009) 174203
[18] P. Borghetti, et al., Langmuir, 26 (2010) 19007
[19] P. Borghetti, et al., J. Chem. Phys., 136 (2012) 204703


Functional properties of nanostructured interfaces and architectures

Among these collaborations, a recent study carried out jointly with the CNR-IOM labs (C. Cepek) has shown the possibility to prepare nanostructured architectures based on ZnO nanorods and carbon, with specific response to polluting gases [20]. Nanostructured materials are also employed to build gas sensing devices with a sub-ppm sensitivity, aimed to bring these devices in the field of environmental monitoring. This is the case of sub-ppm ammonia sensing, achieved by using chemresistor gas sensors built with CNT deposited on plastic substrates [21].

[20] P. Mbuyisaa, et al., Carbon, 50 (2012) 5472
[21] M. Chiesa, et al., J. Environ. Monit., 14 (2012) 1565


Oxide heterointerfaces

Lanthanum aluminate (LaAlO3, LAO in short) and strontium titanate (SrTiO3, STO) are formally band insulators, both being closed-shell compounds (4f0 for LAO and 3d0 for STO), with a band gap of 5.4 and 3.2 eV, respectively. When a LAO-STO heterointerface is created, a p-type heterostructure is obtained if the bulk STO is terminated with a SrO plane (hole doping), while an n-type heterostructure is obtained with a TiO2 plane (electron doping) termination. In the latter case, the LAO-STO interface becomes conducting and yields a quasi-2D electron gas (2DEG). The transition to the metallic state was found to be thickness dependent: the 2DEG is observed only when capping is at least 4 u.c. (unit cell) thick. The main difference between LAO and STO resides in the layer charge polarity: looking at the (001) planes, STO is a non-polar solid, since both Sr-O and Ti-O planes are charge-neutral, while LAO is a polar solid, as it is composed of La-O and Al-O charged layers. The p-type interface is thus formed by matching SrO - AlO2 planes, while the n-type by matching TiO2 - LaO planes.
The observed conductivity is thought to be the response of the system to the diverging potential (the so-called polar catastrophe) created by LAO. As Ti 3d states are on the STO side of the heterojunction, it is clear that STO can host the excess carriers created to avoid the polar catastrophe and the electronic structure of STO can deeply affect the physical properties of the 2DEG, such as effective mass, charge density, and extent of 2D character.
Our research is carried out in collaboration with A. Brinkman and M Hujiben at the Mesa+ Institute (Twente, NL). Through resonant photoemission, we have provided the first experimental evidence that the Ti 3d1 levels [23] related to the 2DEG, while the extent of the 2DEG below the interface, the LAO-STO band offset at the interface, and the origin of charge carriers are discussed in a recent publication [24].

[23] G. Drera, et al., Applied Physics Letters, 98 (2011) 052907
[24] G. Drera, et al., Physical Review B, 87 (2013) 075435