L. Silipigni

Electronic conduction in the layered semiconductor MnPS3

Thermopower, conductivity and photoconductivity measurements as a function of temperature, from 130 to 320 degrees C, have been carried out on manganese thiophosphate, MnPS3. The transport mechanisms involved in distinct temperature ranges and under different illumination conditions have been identified. At 130 degrees C a dark conductivity value of 8.6*10-13 Omega -1 cm-1 has been measured, while photoconductivity values ranged from 8.8*10-13 to 6.1*10-8 Omega -1 cm-1. In particular, the dark conductivity process has been attributed to holes in the phosphorus 3pz valence band. The results have been interpreted on the basis of a model, already used for NiPS3, that assumes a weak, ionic, interaction between the transition-metal and the sulphur atoms. A possible energy distribution of both valence and conduction bands, together with Mn 3d levels, is also provided.

The cadmium seleniophosphate (CdPSe3) XPS and XAES spectra

Abstract The XPS and XAES spectra of the cadmium seleniophosphate (CdPSe 3 ) single crystals have been measured, at room temperature, using Al Kα (1486.6xa0eV) X-rays source. The analyzed core level XPS spectra of the CdPSe 3 compositional atoms have emphasized the absence of non-equivalent atoms in this compound. The cadmium 4d levels show a localized character according to the so-called transition-metal weakly interacting model. As regards the XAES spectra, the selenium L 23 M 45 M 45 and cadmium M 45 N 45 N 45 Auger series have been investigated and the nature of the Cdue5f8Se bond has been analyzed by calculating the Cd modified Auger parameter in CdPSe 3 . The deduced Cd modified Auger parameter shift agrees well with the CdPSe 3 energy bandgap E g .

Optical spectra of the layered Cd2P2S6 and Cd2P2Se6 compounds in the region from 1.6 to 5.5 eV

Room temperature absorption and reflectivity data are presented for Cd2P2S6 and Cd2P2Se6 crystals in the range of photon energies between 1.6 and 5.5 eV. For both compounds the characteristic feature of the optical absorption is that their absorption edge appears to correspond to an indirect allowed transition with an optical energy gap of 3.06 eV for Cd2P2S6 and 2.29 eV for Cd2P2Se6. An associated phonon energy of 74 meV for Cd2P2S6 and 50 meV for Cd2P2Se6 is also deduced. The Cd2P2X6 (with X=S or Se) reflectivity spectra are compared with the early reflectivity spectrum of Zn2P2S6. As observed in previous reflectivity spectra of the Hg2P2X6 compounds, near the fundamental absorption edges the Cd2P2X6 reflectivity spectra show a pronounced rise in reflectivity, a feature common to most layered compounds. For photon energies greater than their absorption edges, the reflectivity spectra of both materials look similar to one another and to that of Zn2P2S6. On the basis of these similarities we have adopted ...

X-ray photoelectron spectroscopy and x-ray excited Auger spectroscopy studies of manganese thiophosphate intercalated with sodium ions

Polycrystalline powders of Na2xMn1−xPS3 have been synthesized from layered MnPS3 material by successive ion-exchange intercalation of potassium and sodium ions. Their x-ray photoelectron spectroscopy (XPS) and x-ray excited Auger spectroscopy spectra have been measured at room temperature using Mgu2009Kα (1253.6 eV) x-ray source. In particular, the Mn, P, and S 2p and Na 1s and 2p core-level regions and the Na Auger KL23L23 transition have been investigated. All the analyzed XPS core-level spectra display a single-peak structure, suggesting the absence of nonequivalent atoms of Na, Mn, P, and S. The manganese XPS spectrum shows, as observed in MnPS3 and in its cesium and potassium intercalation compounds, typical shake-up satellites, suggesting that the Mn–S bond is yet mainly ionic in nature. The comparison with the XPS spectra relative to MnPS3 and its potassium intercalation compound (K2xMn1−xPS3) does not emphasize any relevant difference in the binding energy positions of the investigated core levels, in...

An XPS analysis of the interaction of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin with exfoliated manganese thiophosphate

Composite thin films of (C72H66N8O12S4)yLi2xMn1−xPS3 have been obtained through a solution approach by interacting the tosylate salt of the cationic water soluble 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2T4) and MnPS3 exfoliated in the presence of lithium ions. The thin films have been investigated through x-ray diffraction (XRD), ultraviolet/visible (UV/vis) absorption and mainly x-ray photoemission spectroscopy (XPS). N 1s core-level XPS spectra emphasize the presence of three non-equivalent nitrogen atoms, similarly to the film of the pure H2T4 salt. This result, together with the interlayer spacing determined by the XRD pattern and the evidence from absorption measurements, indicates that the porphyrin is intercalated into MnPS3 layers in a non-protonated form and substantially flattened with respect to the free molecule. The striking likeness between the N 1s core levels in the XPS spectra of the composite material, of the H2T4 salt and of the neutral meso-tetrapyridylporphyrin (H2TPyP) suggests that H2T4 is present between the MnPS3 nanosheets together with its counter-ion (tosylate). This hypothesis is confirmed by the observation of a structure which can be attributed to the sulfur of the counterion in the S 2p core-level XPS spectra of the composite material. An analysis of the Mn 2p and 3p, S and P 2p core-level regions through XPS reveals a strong similarity between the starting MnPS3 and the composite material, suggesting that no charge transfer occurs from the guest (H2T4-tosylate) to the host species (MnPS3).

Spectroscopic studies on manganese thiophosphate intercalated with cesium ions

Cesium ions have been intercalated by ion exchange into layered manganese thiophosphate to produce Cs2xMn1−xPS3. Investigations have been conducted by x-ray diffraction analysis, IR absorption spectroscopy, x-ray photoemission spectroscopy (XPS), and dielectric spectroscopy. The XPS spectra, recorded at room temperature and at the Mn, P, and S 2p, Mn 3p, and Cs 3d and 4d core-level regions, indicate that no charge transfer occurs from guest species (Cs+) to pure host lattice (MnPS3). The presence of shake-up satellites at the Mn 2p and 3p core levels suggests that the cesium intercalation does not alter the nature of the Mn–S bond which is mostly ionic. Dielectric measurements, carried out between 260 and 375K and in the 20Hz–1MHz frequency range, classify Cs2xMn1−xPS3 as hopping charge carrier systems. The nature of such carriers can be hypothesized on the basis of both the intercalation process by cationic substitution and the comparison with the observed dielectric response of the pure MnPS3.

NIR-VIS reflectivity spectra of some transition metal thiophosphates

SummaryRoom temperature optical-reflectivity measurements on some transition metal thiophosphates were carried out in the near infrared and visible regions. The resulting spectra, interpreted on the basis of the «transition metal weakly interacting» model, agree well with earlier optical transmission measurements. Below the fundamental absorption edge, the observed features have been assigned to 3d–3d transitions occurring on the transition metal ion, while those observed at photon energies greater than the absorption threshold have been attributed to transitions from the valence bands to discrete 3d orbital levels or to the conduction bands. A more detailed information on the metal ion 3d levels energy distribution with respect to the valence band states belonging to the (P2S6)4- cluster and a more precise determination of the MPS3 absorption edge energy position have been obtained.

The effects of the lithium intercalation on the X-ray photoelectron spectra of NiPS3

SummaryA detailed XPS study of the lithium-intercalated NiPS3 specimens was performed at the 2p, 3p, 3s core levels of the nickel atoms and at the 2p core levels of the sulphur and phosphorous atoms for various lithium contents. Comparison of the Ni 2p, 3p and 3s XPS spectra corresponding to NiPS3 and LixNiPS3 systems shows some evident trends. In particular, a shift of the Ni main line towards lower binding energies, a decrease in the intensity of the Ni 3p, 2p satellite structures and a change in the full width at half maximum of the Ni 3s band with lithium content are observed. All these findings suggest a change in the 3d electron configuration for high lithium concentrations. As regards the cluster (P2S6)4−, with the addition of lithium, a P 2p main line shift towards higher binding energies is noted, while the S 2p peak shifts towards lower binding energies. These results are discussed in comparison with previous physical measurements concerning the nickel reduction process and the related electronic modifications.

Electronic core levels of layered Li2xMn1−xPS3 film

Lithium intercalate thin films have been obtained from MnPS3 starting material and characterized by the x-ray photoelectron spectroscopy technique. The regions of Mn 2p and 3p, P and S 2p, and Li 1s core levels have been investigated. The analysis of the spectra has revealed the lack of nonequivalent atoms of Mn, P and S, and Li. The presence of shake-up-type satellites at the Mn 2p and 3p core levels suggests that Li2xMn1−xPS3 is a large-gap insulating Mn compound analogously to the parent MnPS3. After lithium intercalation no remarkable shift has been observed in the binding energy positions of the investigated core levels in comparison with the corresponding MnPS3 pure compound. On these bases, we assume that only an ion transfer accompanies the lithium intercalation process.

In-layer conductivity and photoconductivity in MnPSe3, CdPSe3, and CdPS3

Electrical properties of single crystals of the MnPSe3, CdPSe3, and CdPS3 layered compounds have been investigated by in-layer dc conductivity and photoconductivity measurements as a function of temperature. The observed dc conductivity dependence exhibits a semiconductor behavior for all three compounds in spite of their high resistivity values. At high temperatures the dc conductivity of all three compounds shows an Arrhenius temperature dependence, while photoconductivity is thermally activated only for MnPSe3 and almost temperature independent for CdPSe3 and CdPS3. These results, interpreted in terms of the simplified energy-band schemes existing for CdPX3 and MnPSe3, have allowed us to identify the conduction mechanisms and to deduce more information about the distribution of the electronic states in the gap region.