Over the last three decades low-dimensional systems have attracted increasing interest both from the fundamental and technological points of view due to their unique physical and chemical properties. X-ray absorption spectroscopy (XAS) is a powerful tool for the characterization of such kinds of systems, owing to its chemical selectivity and high sensitivity in interatomic distance determination. Moreover, XAS does not require long-range ordering, that is usually absent in low-dimensional systems. Finally, this technique can simultaneously provide information on electronic and local structural properties of the nanomaterials, significantly contributing to clarify the relation between their atomic structure and their peculiar physical properties. This review provides a general introduction to XAS, discussing the basic theory of the technique, the most used detection modes, the related experimental setups and some complementary relevant characterization techniques (diffraction anomalous fine structure, extended energy-loss fine structure, pair distribution function, x-ray emission spectroscopy, high-energy resolution fluorescence detected XAS and x-ray Raman scattering). Subsequently, a selection of significant applications of XAS to two-, one- and zero-dimensional systems will be presented. The selected low-dimensional systems include IV and III–V semiconductor films, quantum wells, quantum wires and quantum dots; carbon-based nanomaterials (epitaxial graphene and carbon nanotubes); metal oxide films, nanowires, nanorods and nanocrystals; metal nanoparticles. Finally, the future perspectives for the application of XAS to nanostructures are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/46/42/423001/pdf

Low-dimensional systems investigated by x-ray absorption spectroscopy: a selection of 2D, 1D and 0D cases

/pdf/materials-characterization-by-synchrotron-x-ray-microprobes-2oc81uvmx4.pdf

Materials characterization by synchrotron x-ray microprobes and nanoprobes

The pure ZnO thin films were deposited by the wet chemistry ('liquid ceramics') method from the butanoate precursors on the single-crystalline (102) sapphire substrates. The films annealed in air (550 °C, 24 h) after butanoate pyrolysis have pronounced texture, and they reveal the ferromagnetic behaviour. Argon annealed films (650 °C, 30min) exhibit randomly oriented grains, where the ferromagnetism of these non-textured films is almost equal to that of bare substrate. In both cases the films consist of dense equiaxial nanograins with size ∼20 nm. We observed that grain boundaries (GBs) and related vacancies are the intrinsic origin for RT ferromagnetism in polycrystals [Straumal et al., Phys. Rev. B 79, 205206 (2009)]. Present results demonstrate that not only the specific area of GBs in nanograined ZnO alone determines the ferromagnetic behaviour of ZnO. The GB character distribution (i.e. GB misorientation and orientation) is different in the textured and non-textured films. Most probably, the GBs with different character possess also different magnetic properties. The role of GBs, free surfaces and interfaces in ferromagnetic behaviour of GaN is discussed. In particular, their presence permits to increase the Mn solubility in GaN without precipitation of secondary ferromagnetic phases.

Influence of texture on the ferromagnetic properties of nanograined ZnO films

The electronic structure of the (Ga,Mn)As, (Ga,Mn)N, and (Ga,Gd)N, diluted magnetic semiconductors (DMSs) were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the $\text{LSDA}+U$ method. The x-ray magnetic circular and linear dichroism (XMCD and XMLD) spectra at the Mn, As, Ga, and $\text{N}\text{ }K$ and Gd, Mn, As, and $\text{Ga}\text{ }{L}_{2,3}$ edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The effect of interstitial Mn atoms was found to be crucial for the x-ray magnetic dichroism at the Mn and $\text{As}\text{ }K$ and ${L}_{2,3}$ edges in the (Ga,Mn)As DMS. The influence of the exchange splitting and spin-orbit coupling strength on each of the constituent atoms were furthermore analyzed. The orientation dependence of the XMCD and XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges in the (Ga,Mn)As DMS was investigated by calculating the XMCD and XMLD spectra for the $⟨001⟩$ and $⟨110⟩$ magnetization axis. We found a quite small anisotropy in the XMCD and a giant anisotropy in the XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges. The exchange splitting of the initial $\text{Mn}\text{ }2p$ core level was found to be responsible for huge magnetocrystalline anisotropy of the XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges. The calculated results are compared with available experimental data.

X-ray magnetic dichroism in III-V diluted magnetic semiconductors: First-principles calculations

Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and \emph{ab initio} studies, that the codoping of GaN:Mn with Mg allows to control the Mn$^{n+}$ charge and spin state in the range $3\le n\le 5$ and $2\ge S\ge 1$. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mg$_k$, where the number of ligands $k$ is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.

/pdf/manipulating-mn-mg-k-cation-complexes-to-control-the-charge-zu7we2fa64.pdf

Manipulating Mn--Mg$_k$ cation complexes to control the charge- and spin-state of Mn in GaN

We report on a characteristic photoluminescence feature of the substitutional Mn in high quality GaMnN layers. The lattice site was identified using atom localization by channeling enhanced microanalysis with a transmission electron microscope. It shows that 96.5%±5.0% of the Mn atoms are incorporated on the substitutional Ga site. In photoluminescence a feature appears at 1.41 eV with a phonon sideband related to the GaN matrix. The temperature evolution is characteristic of an intra-atomic transition and it is assigned to the internal transition E5→T52 of the Mn3+ ion. The assignment is supported by absorption experiments. The persistence of the clear PL signal up to about 1% Mn concentration is proposed to be a fingerprint of high quality diluted GaMnN.

/pdf/intra-atomic-photoluminescence-at-1-41-ev-of-substitutional-131udfngrm.pdf

Intra-atomic photoluminescence at 1.41 eV of substitutional Mn in GaMnN of high optical quality

In this study, we report on the incorporation of dilute Gd amounts into GaN films grown by molecular beam epitaxy. A combination of x-ray fluorescence with x-ray absorption spectroscopic techniques enabled us to examine not only the distribution of rare earth atoms in the GaN matrix but also the short-range structural order. Our results show Gd atoms in a trivalent state with tetrahedral coordination, thus substituting Ga in the wurtzite GaN structure.

X-ray absorption in GaGdN: A study of local structure

With the aim of understanding the observed roomtemperature giant magnetic moment of highly diluted GaGdN reported in literature GaGdN, GaGdN:Si and GaGdN:H have been grown by molecular beam epitaxy (MBE) and the Gd concentration is between 1016 and 1021 cm3. All samples are ferromagnetic at 300 K as measured by SQUID and the very high saturation magnetic moment in this type of samples is confirmed by our experiments.



With co-doping of hydrogen or silicon we were able to decrease or increase the strength of the magnetic moments in the layers, respectively. All our findings indicate that ferromagnetism is stabilized by electrons. Electrical measurements on GaGdN layers grown on highly resistive 6H-SiC (0001) show a typical temperature dependence of hopping conductivity. The resistivity of the material is 0,1 to 13 kΩ cm in the temperature range from 60 to 10 K. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron stabilized ferromagnetism in GaGdN

GaMnN layers have been grown by MBE on Si(111) in a wide range of growth conditions. For different substrate temperatures the structural and composition dependence of the grown layers has been studied as a function of the Mn supply. We find that at regular substrate temperature the incorporation of Mn into the layer is low and difficult to control. Only at a reduced substrate temperature of 650 °C and under N-rich growth conditions it is possible to grow homogeneous GaMnN layers if the Mn supply is below 15% of the total metal flux. The incorporation efficiency in this range is about 30%, which corresponds to a maximum Mn content in the diluted layers of about 5%. Above a critical Mn supply, GaMn3N precipitates are formed, which often extend out of the surface with a typical pyramidal structure.

Mn incorporation in GaN thin layers grown by molecular-beam epitaxy

Imaging of extended defects at atomic resolution using high-resolution transmission electron micrography (HRTEM) is problematic due to the lens aberrations of an uncorrected microscope and the loss of the phase during recording of images. As a result, techniques to measure the strain fields of these defects can only be applied under distinct and well-controlled conditions sometimes in a narrow window of specimen thickness and imaging conditions. Using a combination of focal series reconstruction and numerically correction of the lens aberrations a-posteriori allows overcoming these limitations. Also the strain fields can be measured using geometrical phase analysis without further precaution. We will demonstrate the application of these techniques on extended defects found in GaN. A discrepancy between the theoretically predicted structure of the Ga-terminated 90°-partial and the experimentally observed one has been put in evidence. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High resolution imaging of extended defects in GaN using wave function reconstruction

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