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Adsorption and dissociation of H2O on monolayer MoS2 surface doped with B/Si explored by first principle calculations coupled with climb image nudged elastic band method. The impurity B/Si atoms are more likely to occupy sulfur atom lattice positions with lower formation energies, which results in the breaking of surface chemically inertness of the MoS2 (001) plane. Free H2O molecules can be adsorbed on doped surfaces with lower adsorption energies, and form chemical bonds between oxygen and impurity atoms. Adsorbed H2O can be separated into OH radical and hydrogen adatom with a lower dissociation energy barrier on these doped systems (including impurity B/Si single doped, B and Si co-doped). Furthermore, the adsorbed OH radical can be further dissociated into oxygen and hydrogen adatoms on the surface of B and Si co-doped MoS2. Our theoretical study suggests that substrate of monolayer MoS2 doped with impurity B, Si may be a potential alternative choice for hydrogen evolution reaction.

Porous silica SBA-15 was synthesized using triblock copolymer P123 as structure template and tetraethyl orthosilicate (TEOS, C8H20O4Si) as silicon source. The pore structure was adjusted by using swelling agent 1,3,5-trimethylbenzene (TMB). Small angle X-ray scattering (SAXS) and high resolution transmission electron microscopy (HRTEM) indicate that the pore morphology changes in three stages with increasing TMB/P123 ratio. In the first stage (TMB/P123 τ 3 and τ 4, which are due to o-Ps annihilation in micropores and mesopores, respectively. It was found that the mesopores are open pores and the micropores are closed pores. With the increase of TMB ratio, the number and size of micropores show nearly no change, but the mesopore size increases gradually with increasing TMB/P123 ratio up to 0.5, which are in agreement with the results of the N2 adsorption/desorption measurements. However after the pores evolves into mesoporous foam, the pore size obtained from o-Ps lifetime is less than that of N2 adsorption/desorption measurements.

High purity $$\hbox {In}_2\hbox {O}_3$$ nanopowders were subjected to different thermal treatments to investigate the role of defects on the lattice thermal conductivity. The powders were first treated by spark plasma sintering (SPS) at $$700\,^{\circ}\hbox {C}$$ and annealed in air between 700 and $$1300\,^{\circ}\hbox {C}$$ . X-ray diffraction measurements show that the samples are single phase, and the diffraction peaks become sharper with increasing of annealing temperature, indicating improvement in crystallinity and increase in grain size. The $$\hbox {In}_2\hbox {O}_3$$ nanopowders were also treated by SPS sintering at different temperatures without subsequent annealing. On the contrary, the average grain size of $$\hbox {In}_2\hbox {O}_3$$ treated by SPS has no obvious change with the increase in sintering temperatures. Positron annihilation measurements reveal large amounts of monovacancies and vacancy clusters in the $$\hbox {In}_2\hbox {O}_3$$ nanocrystals. The monovacancies gradually recover and the vacancy clusters transform into smaller vacancies with increasing annealing or sintering temperatures. The lattice thermal conductivity increases with the increase in annealing or sintering temperature, which shows close correlation with the recovery of vacancy defects after heat treatment. This gives us strong evidence that vacancy defects play an important role on the suppression of lattice thermal conductivity in nanostructured thermoelectric materials.

The stability of various defects in monolayer MoS 2 , as well as their interactions with free O 2 molecules were investigated by density functional theory (DFT) calculations coupled with the nudged elastic band (NEB) method. The defects including S vacancy (monosulfur and disulfue vacancies), antisite defect (Mo S ) and external Mo atom can exist steadily in monolayer MoS 2 , and introduce defect levels in these defective systems, which breaks the surface chemical inertness and significantly enhances the adsorption capacity for free O 2 . The adsorption energy calculations and electronic properties analysis suggest that there is a strong interaction between O 2 molecule and defective system. The adsorbed O 2 on the defective surface can dissociate with a lower activation energy barrier, which produce two active oxygen atoms. Especially, two Mo atoms can occupy one Mo lattice site, and adsorbed O 2 on the top of the Mo atom can then dissociate directly with the lowest activation energy barrier. Hence, our work may provide useful information to design MoS 2 -based gas sensor or catalysts.

Co $$^+$$ ions with multiple energies from 50 to 380 keV were implanted into ZnO single crystals up to a total dose of $$1.25\times 10^{17}\,\hbox {cm}^2$$ . The implanted samples were annealed in open air for 30 min between 200 and 1100 $$^{\circ }$$ C. All the samples before and after implantation and annealing were characterized by X-ray diffraction (XRD), Raman scattering and positron annihilation measurements. XRD and Raman scattering measurements indicate that Co implantation induces severe lattice damage, and after annealing the damage recovers gradually. No Co clusters or Co-related second phase was observed in the implanted samples. Doppler broadening of positron annihilation radiation measurements using a slow positron beam reveals a large number of vacancy clusters introduced by Co implantation. After annealing up to 1000 $$^{\circ }$$ C, almost all the defects induced by implantation are removed. The implanted samples show clear ferromagnetism measured at 5 K. It shows very slight decrease after annealing at 700 $$^{\circ }$$ C and becomes much weaker after annealing at 1000 $$^{\circ }$$ C. The origin of ferromagnetism is most probably due to substitution of Co $$^+$$ ions at Zn lattice sites. However, it is apparent that the decrease in magnetization after annealing is consistent with the vacancy recovery process, indicating that the ferromagnetism in Co-implanted ZnO is mediated by defects such as Zn vacancy (V $$_{Zn}$$ ) or vacancy clusters. First principles calculations also support that Zn-related monovacancies and vacancy clusters can enhance the ferromagnetism in Co-doped ZnO.

Adsorption of small gas molecules (O2, NO, NO2 and NH3) on transition-metal Cu atom embedded monolayer MoS2 was investigated by first-principles calculations based on the density-functional theory (DFT). The embedded Cu atom is strongly constrained on the sulfur vacancy of monolayer MoS2 with a high diffusion barrier. The stable adsorption geometry, charge transfer and electronic structures of these gas molecules on monolayer MoS2 embedded with transition-metal Cu atom are discussed in detail. It is found that the monolayer MoS2 with embedded Cu atom can effectively capture these gas molecules with high adsorption energy. The NH3 molecule acts as electron donor after adsorption, which is different from the other gas molecules (O2, NO, and NO2). The results suggest that MoS2-Cu system may be promising for future applications in gas molecules sensing and catalysis, which is similar to those of the transition-metal embedded graphene.

Positron lifetime and Doppler broadening of positron annihilation radiation were measured as a function of time to study the irradiation effect by 22Na positron source in fine powdered alumina. The γ-Al2O3 samples were put in a vacuum chamber with a pressure of about 10−6 Torr and were cooled down to 10 K by a closed-cycle helium gas refrigerator. The irradiation of γ-Al2O3 samples by positron source was taken for a duration of about two days immediately after the sample was cooled down. After that, the sample was subjected to a warm up process from 10 K to 300 K with a step of 10 K. Positron lifetime and Doppler broadening spectra were measured simultaneously during these processes. Two long lifetime components corresponding to ortho-positronium annihilation were observed. A significant shortening of these long lifetime components and a large increase in S parameter is observed during irradiation. It is supposed that positron source irradiation creates a large number of paramagnetic centers on the surface of the γ-Al2O3 grains, which induce spin conversion quenching of positronium. The irradiation induced paramagnetic centers are unstable above 70 K and are nearly annealed out when the temperature rises to 190 K. After warming up of the sample to room temperature, the positron lifetime spectrum is identical to that before irradiation. It was also found that after irradiation, a medium long lifetime component of about 5 ns appears, of which the intensity increases with increasing irradiation time. This may be originated from the formation of the surface o-Ps state. This surface o-Ps state is also inhibited at elevated temperatures. Our results indicate that positronium is a very sensitive probe for the surface defects in porous materials.

Mesoporous silica with ordered pore structure was synthesized at various temperatures using TEOS as the silica source and P123 as the template. Both small angle X-ray scattering (SAXS) and high resolution transition electron microscopy (HRTEM) measurements verify the ordered pore structure of the synthesized SiO 2 . With synthesis temperature increasing, the pore structure has slight damage at 130 °C, while it shows ordered structure again at 150 °C. When the synthesis temperature increases to 180 °C, the ordered pore structure is severely destructed. The change of pore structure is further confirmed by scanning electron microscopy (SEM) measurements. Positron lifetime measurements reveal four lifetime components in the synthesized mesoporous SiO 2 , and the two long lifetimes τ 3 and τ 4 correspond to the annihilation of o-Ps in the micropores and large pores of the material, respectively. The longest lifetime τ 4 tends to increase slightly with increasing synthesis temperature. However, its intensity I 4 shows an overall decrease with exception at 150 °C. At the synthesis temperature of 180 °C, the intensity I 4 decreases drastically to about 17.5%. This indicates variation of the size and fraction of pores with increasing synthesis temperature, and the pore structure is seriously destructed at 180 °C. By comparing with the N 2 adsorption–desorption measurements, it was found that the Goworek's model is more suitable for the size estimation of cylindrical pores from the o-Ps lifetime, while Dull's and Ito's model is appropriate for the rectangular and spherical pores, respectively.

Clathrate Ba 6 Ge 25 is prepared by melt method and spark plasma sintering. Structural transition below room temperature is studied by positron annihilation and X-ray diffraction measurements. There is a pronounced transition in the temperature range of 200–250 K which might be involved with the movement of Ba atoms in Ge cages and result in disordered structure. This transition is further confirmed by the theoretical calculation of positron annihilation states. Thus our results confirm the structural models proposed by Carrillo-Cabrera et al. (2005). The measured specific heat capacity, electric resistivity and magnetic susceptibility all show anomalous transition in the same temperature range, indicating that the movement of Ba atoms in the cage has influence on the thermal, electric as well as magnetic properties of Ba 6 Ge 25 .

Commercial Al–Mg alloy AA5083 was severely deformed with thickness reduction from 50 mm to 1 mm by hot rolling plus cold rolling. Isochronal annealing studies of this heavily deformed Al–Mg alloy were investigated by positron annihilation and Vickers microhardness as well as X-ray diffraction measurements. Positrons are almost fully trapped at dislocations in the as deformed sample. This suggests that deformation introduces large amounts of dislocations. The Vickers microhardness also increases from 71 Hv to 155 Hv after deformation. Most of the deformation induced dislocations are annealed out at 250 °C. At the same time, the microhardness also decreases to about 90 Hv after annealing at 200–250 °C and remain stable after further annealing up to 500 °C. This indicates that hardening of the AA5083 alloy is primarily due to dislocations. Dislocation-solute interaction may be also responsible for the strengthening of hardness.

CoO/Al 2 O 3 catalysts were prepared by wet impregnation method with CoO contents ranging from 0 wt% to 24 wt%. X-ray diffraction and X-ray photoelectron spectroscopy measurements suggest formation of CoO after calcined in N 2 . Quantitative X-ray diffraction analysis indicates monolayer dispersion capacity of CoO in CoO/Al 2 O 3 catalysts to be about 3 wt%. Positron annihilation lifetime and coincidence Doppler broadening measurements were performed to study the dispersion state of CoO on Al 2 O 3 . The positron lifetime measurements reveal two long lifetime components τ 3 and τ 4 , which correspond to ortho -positronium annihilation lifetime in microvoids and large pores, respectively. It was found that the positronium atom is very sensitive to the dispersion state of CoO on Al 2 O 3 . The presence of CoO significantly decreases both the lifetime and the intensity of τ 4 . Detailed analysis of the coincidence Doppler broadening measurements suggests that with the CoO content lower than the monolayer dispersion, spin conversion reaction of positronium is induced by CoO. When the cobalt content is higher than the monolayer dispersion capacity, inhibition of positronium formation becomes the dominate effect.

Two mesoporous In 2 O 3 were prepared with different pore size by using KIT-6 template synthesized at temperature of 50 ° C and 100 ∘ C (designated as 50 ° C– In 2 O 3 and 100 ∘ C– In 2 O 3 ), respectively. Small angle X-ray scattering and transmission electron microscope measurements all confirm ordered pore structure in the synthesized In 2 O 3 . N 2 adsorption/desorption analysis indicates that the pore size in these two In 2 O 3 is about 11.06 nm and 8.737 nm, respectively. Positron lifetime measurements suggest existence of both micropores and mesopores in the porous In 2 O 3 samples, and the mesopore size in 100 ∘ C– In 2 O 3 is smaller than that in 50 ∘ C– In 2 O 3 . The porosity of 50 ° C– In 2 O 3 and 100 ∘ C– In 2 O 3 is about 44% and 40%, respectively. Comparing with the bulk In 2 O 3 obtained by SPS sintering of the commercial In 2 O 3 nanopowders at high temperature of 900 ∘ C, the thermal conductivity of porous In 2 O 3 decreases by more than an order of magnitude. For the 100 ∘ C– In 2 O 3 , the thermal conductivity is as low as 0.58 W m − 1 K − 1 at room temperature of 25 ∘ C, which is below the amorphous limit. On the other hand, the electrical conductivity of the porous In 2 O 3 is deteriorated by the pore structure, but it is partially compensated by the increase of Seebeck coefficient. The overall ZT factor increases to higher than 0.08 at 300 ∘ C for 50 ∘ C– In 2 O 3 , which is almost three times that of the bulk In 2 O 3 .

Li-doped ZnO polycrystals with Li2CO3 concentration between 0 and 15 at.% have been prepared using a solid state reaction method. After annealing of the mixed samples at 1200 °C, Li2CO3 completely decomposed to Li2O and the lattice parameters a/c first increase with Li2CO3 concentration up to 5 at.%, and then decrease with increasing Li2CO3 concentration. This indicates that Li atoms prefer interstitial sites at low concentration and substitute Zn sites when Li concentration is beyond a certain threshold. Positron annihilation measurements were performed for undoped ZnO and Li2CO3-doped ZnO with 15 at.% samples as a function of annealing temperature. A high concentration of vacancy defects is observed in the undoped and as-prepared Li2CO3-doped samples, and they are fully removed after annealing at 1200 °C. The average positron lifetimes of all Li2CO3-doped samples after annealing at 1200 °C are the same, suggesting that Li doping has little effect on the positron lifetime in ZnO. This might be due to the invisibility of positron to Lii or LiiLiZn complex, which is further confirmed by theoretical calculation.

NiO/ γ -Al 2 O 3 catalysts with NiO content of 9 wt% and 24 wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100 °C to 1000 °C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ 3 and τ 4 , which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ 4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ 4 and its intensity I 4 , which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600 °C, abnormal increase of the lifetime τ 4 is observed. This is due to the formation of NiAl 2 O 4 spinel from the reaction of NiO and γ -Al 2 O 3 . The generated NiAl 2 O 4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ 4 . Formation of NiAl 2 O 4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.

High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400 °C. Variation of the microstructures was investigated by X-ray diffraction and positron annihilation spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60 nm with temperature increasing up to 1400 °C. Positron annihilation measurements reveal vacancy defects including Mg vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Mg monovacancies and vacancy clusters was observed after annealing above 1200 °C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000 °C. However, after 1400 °C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects.

High purity BaTiO3 nanopowders were pressed into pellets and annealed between 100 and 1200 °C. The crystal quality and grain size of the BaTiO3 nanocrystals were characterized by X-ray diffraction measurements. Annealing induces an increase in the grain size from 44 to 82 nm with temperature increasing up to 1200 °C. XRD and Raman spectroscopy studies confirm that all the samples were single phase with a tetragonal structure after annealing at different temperatures. Positron annihilation measurements reveal large number of vacancy defects in the grain boundary region. These interfacial defects remain stable after annealing at temperatures below 400 ° C and begin to disappear rapidly above 700 °C. After annealing at 1200 °C, most of the interfacial defects have been removed. Hysteresis loops are observed for the 100 ° C annealed samples, which indicate ferromagnetism in BaTiO3 nanocrystals. The ferromagnetism becomes a little weaker after annealing at 700 °C, and it disappears after 1200 ° C annealing. This change of ferromagnetism coincides with the defect recovery process after annealing, suggesting that ferromagnetism might originate from the interfacial defects.

Fe ions were implanted into ZnO single crystals with multiple energies between 50 and 380 keV up to a total fluence of 12.5 × 1 0 17 cm−2. The crystal quality of Fe+ implanted ZnO was investigated by X-ray diffraction 2 θ scans and ω -rocking curve measurements. The appearance of Fe related diffraction peaks after 700 ° C annealing of the implanted sample indicates possibly formation of Fe nanoparticles. Superconducting quantum interference device measurements revealed ferromagnetic behavior below 250 K for both the as-implanted and post-annealed ZnO. Photoluminescence and Raman scattering as well as X-ray rocking curve measurements all indicate introduction of structural defects after Fe implantation. Some of the defects act as nonradiative recombination centers, and suppress the visible and ultraviolet (UV) emission in ZnO. These defects also break the Raman selection rule, and lead to the activation of some silent phonon modes. Annealing of the implanted sample at 700 ° C causes partial recovery of the defects, however, the X-ray diffraction results of the anneal ZnO show even an improvement of the crystal quality compared with the un-implanted one. This could be attributed to the recovery of the grown-in defects in ZnO.

In this work, we implanted B+, O+, Al+, and P+ ions into ZnO with energy of 50-380 keV and total doses of 4 × 1015 cm–2 for each ion. The implantation-induced defects and their thermal recovery were studied using a slow positron beam. Vacancy clusters are produced in all the implanted samples. It is found that the thermal recovery of these vacancies induced by different ions shows much difference. In case of B+ and Al+-implantation, the vacancy clusters agglomerate to much larger size and might evolve to microvoids during annealing. However, for O+ and P+ ions, which are heavier than B+ and Al+, the vacancies show a much weaker agglomeration process. The mechanism of such difference is discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The epoxy-rectorite nanocomposites were prepared at different curing temperature ranged from 70 °C to 200 °C and the effects of curing temperature on microstructure of material have been studied by positron annihilation lifetime spectroscopy (PALS), and X-ray diffraction (XRD). The XRD patterns show that the intercalated structure existed in the samples cured at 70 °C and 90 °C, and the clay platelets were completely exfoliated while the curing temperature is higher than 110 °C. At low rectorite content the Ps is mainly formed and annihilated in the free volume of the epoxy matrix and the average size of the free volume is essentially unchanged with the curing temperature. With the increasing curing temperature from 70 °C to 110 °C, the dramatic increase in the intensity I2 of second lifetime component indicates the formation of interfacial layers between rectorite and epoxy matrix. The distributions of o-Ps lifetime and the free volume size have the narrowest distribution for the samples cured at temperature 110 °C and 140 °C, which imply the most homogeneous structure existed in the nanocomposites. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, the numerical Laplace inversion technique and maximum entropy method are utilized to extract continuous positron lifetime distribution in semiconductors. The result is used to discriminate the native vacancy-type defects in as-grown GaAs and InP with different conduction type. Direct evidence of shallow positron traps were also observed in ion-implanted p-InP. It is demonstrated that the lifetime distribution can give us more detailed information on the native defects.

Positron lifetime spectra have been measured on plastically deformed Zn-doped p-type GaAs with strain ranging from 0 to 15% at room temperature. The lifetime spectra were analyzed using PATFIT and MELT programs. Two lifetime components were observed in deformed samples. The second lifetime τ 2 ∼ 400 to 450 ps indicated that vacancy clusters were formed during deformation. From the temperature behavior of positron lifetime in 15% deformed samples, the positron detected negative ions as shallow positron traps, and the dominant shallow traps were attributed to acceptor impurity Zn ions with negative charge. The positron trapping rate of vacancy clusters was also estimated.

In this paper, two methods of extracting continuous positron lifetime distribution are utilized, one is the numerical Laplace inversion, another is the maximum entropy method. The result of continuous lifetime distribution is used to discriminate the native vacancy-type defects in GaAs and InP with different conduction type. It is demonstrated that the lifetime distribution can give us more detailed information on the native defects. In particular, the direct evidence of positron trapping by vacancies is observed both in semi-insulating GaAs and in p-type InP, of which the identification of defects is still a question in debate.

A series of GaAs/InAs/GaAs samples were studied by double crystal X-ray diffraction and the X-ray dynamic theory was used to analyze the X-ray diffraction results. As the thickness of InAs layer exceeds 1.7 monolayer, 3-dimensional InAs islands appear. Pendellosung fringes shifted. A multilayer structure model is proposed to describe the strain status in the InAs islands of the sample and a good agreement is obtained between the experimental and theoretical curves.

The positron lifetime spectra have been measured in three semi-insulating (SI) GaAs samples, one n-type GaAs doped with Te, and one p-GaAs doped with Zn. The average lifetimes in all SI-GaAs samples were 229 ps, and were larger than the bulk lifetime found in p-GaAs. Moreover, the annihilation rate distribution (ARD) in SI-GaAs was much wider than that in p-type GaAs. This showed that gallium vacancies existed in SI-GaAs. By measuring its Doppler broadening spectra as a function of temperature between 10 and 300 K, we observed that the S-parameter decreased with temperature increasing from 10 to 120 K. Thus the charge state of V Ga was determined to be negative. Another defect V As in SI-GaAs has also been detected under photoexcitation at 77 K by the increase of the S-parameter with the driving current in GaAs LED which was not observed in the dark.

Positron lifetime spectra are measured for X-type synthetic zeolite (10X and 13X). By exposing zeolite 13X to air with pressure ranging from 0 to 100 kPa, spin-conversion of o-Ps is observed and the two longer lifetimes τ 3 and τ 4 are found to be associated with o-Ps annihilation in the β cage and supercage, respectively. For calcined zeolite 10X, the lifetime spectra are measured at temperatures from 25 to 750 o C. Our results show that with the desorption of water molecules from cages, its free volume becomes larger. Moreover, when most of the water molecules are removed from the zeolite framework, Bronsted acids are observed both in the β cage and in the supercage. With the calcination temperature getting even higher (above 700 o C) the framework structure ofzeolite begins to break down. A detailed discussion is presented

Defects in hydrothermal grown ZnO single crystals are studied as a function of annealing temperature using positron annihilation, x-ray diffraction, Rutherford backscattering, Hall, and cathodoluminescence measurements. Positron lifetime measurements reveal the existence of Zn vacancy related defects in the as-grown state. The positron lifetime decreases upon annealing above 600 °C, which implies the disappearance of Zn vacancy related defects, and then remains constant up to 900 °C. The Rutherford backscattering and x-ray rocking curve measurements show the improvement of crystal quality due to annealing above 600 °C. Although the crystal quality monitored by x-ray diffraction measurements is further improved after annealing at above 1000 °C, the positron lifetime starts to increase. This is due to either the formation of Zn vacancy related defects, or the change of the Zn vacancy charge state occupancy as a result of the Fermi level movement. The electron concentration increases continuously with increasing annealing temperature up to 1200 °C, indicating the formation of excess donors, such as oxygen vacancies or zinc interstitials. The cathodoluminescence measurements reveal that the ultraviolet emission is greatly enhanced in the same temperature range. The experimental results show that the ZnO crystal quality, electrical and optical characteristics are improved by postgrowth annealing from 600 to 1200 °C. The disappearance of Zn vacancy related defects contributes to the initial stage of improved crystal quality.

The structural instabilities and charge transfer in cuprate superconductors have been studied by positron annihilation spectroscopy (PAS). The results of positron annihilation parameters were presented as a function of temperature from 77 to 300 K in a variety of high-Tc superconductors. The anomalies of positron lifetime were observed at some temperature regions above Tc and this phenomenon reflects the structural instabilities at normal state. The difference of Doppler broadening energy spectra of annihilation radiation in normal and superconducting states shows an increase of electron density with high momentum and the results are interpreted in terms of charge transfer between the Cu-O and Bi-O planes.

MBE is the most highly developed technique to prepare ultra-thin multilayer heterostructures. In this paper, we report that high-quality quantum well and superlattice structures with controllable layer thicknesses as thin as one monolayer and with ideally abrupt GaAs A l x Ga 1− x As interfaces have been obtained by a home-made MBE system and investigated from the view-point of crystallography and optical properties.