Thermal Sublimation Research Articles

Investigation of Physical and Electronic Properties of GeSe for Photovoltaic Applications

AbstractGeSe is a promising absorber material for photovoltaic applications due to its attractive material, optical, and electrical properties as well as its low toxicity and earth abundance. The first GeSe‐based solar cell with 1.48% efficiency has been recently reported through self‐regulated rapid thermal sublimation. However, most of the fundamental physical and electronic properties of GeSe such as refractive index, dielectric constant, carrier mobility, lifetime, and diffusion length remain unclear, despite the necessity of this for the design of high‐performance GeSe solar cells. In this work, the above basic properties of GeSe are systematically studied by using spectroscopic ellipsometry, space charge limited current measurements, and transient absorption spectroscopy. These comprehensive results provide a solid foundation for the further development of GeSe solar cells.

Thermal sublimation: a scalable and controllable thinning method for the fabrication of few-layer black phosphorus

We report uniform layer-by-layer sublimation of black phosphorus under heating below 600 K. The uniformity and crystallinity of BP samples after thermal thinning were confirmed by Raman spectra and Raman mapping. The sublimation rate of BP was around 0.18 nm min−1 at 500 K and 1.15 nm min−1 at 550 K. Both room and high temperature Raman peak intensity ratio as functions of BP thickness were established for in situ thickness determination and control. Uniform and crystalline 2 to 4-layer BP flakes with areas from 10 to 1000 μm2 were prepared with this method. No micron scale defects were observed. The sublimation thinning method was shown to be a controllable and scalable approach to prepare high-quality few-layer black phosphorus.

Ray craters on Ganymede: Implications for cratering apex-antapex asymmetry and surface modification processes

As the youngest features on Ganymede, ray craters are useful in revealing the sources of recent impactors and surface modification processes on the satellite. We examine craters with D> 10km on Ganymede from images obtained by the Voyager and Galileo spacecraft to identify ray craters and study their spatial distributions. Furthermore, we carefully select images of appropriate solar and emission angles to obtain unbiased ray crater densities. As a result, we find that the density of large ray craters (D> 25km) on the bright terrain exhibits an apex-antapex asymmetry, and its degree of asymmetry is much lower than the theoretical estimation for ecliptic comets. For large craters (D> 25km), ecliptic comets ought to be less important than previously assumed, and a possible explanation is that nearly isotropic comets may play a more important role on Ganymede than previously thought. We also find that small ray craters (10km<D < 25km) on the bright terrain and ray craters (D> 10km) on the dark terrain show no apex-antapex asymmetry. We interpret that the distribution difference between the terrain types comes from preferential thermal sublimation on the dark terrain, while the distribution difference between large and small ray craters suggests that rays of small craters are more readily erased by some surface modification processes, such as micrometeorite gardening.

High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient optical micro-imaging technique which can be used to map and quantify such behavior. Europium thenoyltrifluoroacetonate (EuTFC) has a 612 nm luminescence line whose activation efficiency drops strongly with increasing temperature, due to T-dependent interactions between the Eu3+ ion and the organic chelating compound. This material may be readily coated on to a sample surface by thermal sublimation in vacuum. When the coating is excited with ultraviolet light (337 nm) an optical micro-image of the 612 nm luminescent response can be converted directly into a map of the sample surface temperature. This technique offers spatial resolution limited only by the microscope optics (about 1 micron) and time resolution limited by the speed of the camera employed. It offers the additional advantages of only requiring comparatively simple and non-specialized equipment, and giving a quantitative probe of sample temperature.

Structure, magnetic properties and thermal sublimation of fluorinated Fe4 Single-Molecule Magnets

Fluorinated tetrairon(III) Single-Molecule Magnets (SMMs) [Fe4(L1)2(dpmF6)6] (1dpmF6), [Fe4(L2)2(dpmF6)6] (2dpmF6), and [Fe4(L2)2(pta)6] (2pta) were assembled combining the tripodal ligands H3L1=2-hydroxymethyl-2-phenylpropane-1,3-diol and H3L2=S-5-hydroxy-4,4-bis(hydroxymethyl)pentyl ethanethioate with fluorinated β-diketones analogues of dipivaloylmethane (Hdpm), namely 1,1,1-trifluoro-2,6,6-trimethyl-2-(trifluoromethyl)heptane-3,5-dione (HdpmF6) and pivaloyltrifluoroacetone (Hpta). The new compounds, along with [Fe4(L1)2(dpm)6] (1dpm) and [Fe4(L1)2(pta)6] (1pta), were designed in order to investigate the effect of fluorination degree on processability by thermal sublimation. The two different functional groups on the tripodal ligands, i.e. C6H5 in H3L1 and (CH2)3SAc in H3L2, are suitable for promoting physisorption and chemisorption on surfaces, respectively. Direct current magnetic data are typical for the metal-centred triangular topology of Fe4 complexes, with antiferromagnetic nearest-neighbour coupling constants in the range 16–18cm−1 and an S=5 ground spin state. Alternating current susceptibility measurements showed that slow magnetic relaxation persists in fluorinated compounds. When heated in high vacuum (10−7mbar), 2dpmF6 and 2pta undergo thermal decomposition before subliming, while 1dpmF6 was found to sublimate at 497±5K in the same conditions, being the third sublimable SMM of this family after 1pta (440±5K) and 1dpm (500±10K).

Quantitative effects of rapid heating on soot-particle sizing through analysis of two-pulse LII

During the rapid laser pulse heating and consecutive cooling in laser-induced incandescence (LII), soot particles may undergo thermal annealing and sublimation processes which lead to a permanent change in its optical properties and its primary particle size, respectively. Overall, effects of these two processes on soot and LII model-based particle sizing are investigated by measuring the two-color time-resolved (2C-TiRe) LII signal decay from in-flame soot after two consecutive laser pulses at 1064-nm wavelength. Experiments are carried out on a non-premixed laminar ethylene/air flame from a Santoro burner with both low and moderate laser fluences suitable for particle sizing. The probe volume is set to a radial position close to the flame axis where the soot particles are known to be immature or less graphitic. With the first pulse, soot is pre-heated, and the LII signal after the consecutive second pulse is used for analysis. The two-color incandescence emission technique is used for the pyrometric determination of the LII-heated peak soot temperature at the second pulse. A new LII simulation tool is developed which accounts for particle heating via absorption and annealing, and cooling via sublimation, conduction, and radiation with various existing sub-models from the literature. The same approach of using two laser pulses is implemented in the simulations. Measurements indicate that thermal annealing and associated absorption enhancement becomes important at laser fluences above 0.17 J/cm2 for the immature in-flame soot. After a heating pulse at 0.33 J/cm2, the increase of the soot absorption function is calculated as 35% using the temperature measured at the second pulse and an absorption model based on the Rayleigh approximation. Present annealing model, on the other hand, predicts graphitization of soot even in the absence of laser heating at typical flame temperatures. Recorded experimental LII signal decays and LII-heated peak soot temperature information are used for particle sizing with the LII modeling to assess the effects of sublimation. A reduction in particle size due to sublimation starts at a laser fluence of 0.1 J/cm2 for the in-flame soot. After a heating pulse at 0.33 J/cm2, the particle loses 55% of its initial mass.

GeSe Thin-Film Solar Cells Fabricated by Self-Regulated Rapid Thermal Sublimation.

GeSe has recently emerged as a promising photovoltaic absorber material due to its attractive optical and electrical properties as well as earth-abundant and low-toxic constituent elements. However, no photovoltaic device has been reported based on this material so far, which could be attributed to the inevitable coexistence of phase impurities Ge and GeSe2, leading to detrimental recombination-center defects and seriously degrading the device performance. Here we overcome this issue by introducing a simple and fast (4.8 μm min-1) rapid thermal sublimation (RTS) process designed according to the sublimation feature of the layered structured GeSe. This new method offers a compelling combination of assisting raw material purification to suppress deleterious phase impurities and preventing the formation of detrimental point defects through congruent sublimation of GeSe, thus providing an in situ self-regulated process to fabricate high quality polycrystalline GeSe films. Solar cells fabricated following this process show a power conversion efficiency of 1.48% with good stability. This preliminary efficiency and high stability, combined with the self-regulated RTS process (also extended to the fabrication of other binary IV-VI chalcogenide films, i.e., GeS), demonstrates the great potential of GeSe for thin-film photovoltaic applications.

Morphology-defined interaction of copper phthalocyanine with O2/H2O

Copper phthalocyanine (CuPc) is an important hole transport layer for organic photovoltaics (OPVs), but interaction with ambient gas/vapor may lead to changes in its electronic properties and limit OPV device lifetimes. CuPc films of thickness 25 and 100 nm were grown by thermal sublimation at 25°C, 150°C, and 250°C in order to vary morphology. We measured electrical resistance and film mass in situ during exposure to controlled pulses of O2 and H2O vapor. CuPc films deposited at 250°C showed a factor of 5 higher uptake of O2 as detected by a quartz crystal microbalance (QCM), possibly due to the formation of β-CuPc at T>200°C which allows higher O2 mobility between stacked molecules. While weight-based measurements stabilize after ∼10 min of gas exposure, resistance response stabilizes over times >1 h, suggesting that mass change occurs by rapid adsorption at active surface sites whereas resistive response is dominated by slow diffusion of adsorbates into the bulk film. The 25 nm films exhibit higher resistive response than 100 nm films after an hour of O2/H2O exposure due to fast analyte diffusion down to the film/electrode interface. We found evidence of decoupling of CuPc from the gold-coated QCM crystal due to preferential adsorption of O2/H2O molecules on gold.

Robust Magnetic Properties of a Sublimable Single-Molecule Magnet.

The organization of single-molecule magnets (SMMs) on surfaces via thermal sublimation is a prerequisite for the development of future devices for spintronics exploiting the richness of properties offered by these magnetic molecules. However, a change in the SMM properties due to the interaction with specific surfaces is usually observed. Here we present a rare example of an SMM system that can be thermally sublimated on gold surfaces while maintaining its intact chemical structure and magnetic properties. Muon spin relaxation and ac susceptibility measurements are used to demonstrate that, unlike other SMMs, the magnetic properties of this system in thin films are very similar to those in the bulk, throughout the full volume of the film, including regions near the metal and vacuum interfaces. These results exhibit the robustness of chemical and magnetic properties of this complex and provide important clues for the development of nanostructures based on SMMs.

Role of impurities in determining the exciton diffusion length in organic semiconductors

The design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length (LD). Despite the importance of this parameter, there have been few investigations connecting LD and materials purity. Here, we investigate LD for the organic small molecule N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine as native impurities are systematically removed from the material. Thin films deposited from the as-synthesized material yield a value for LD, as measured by photoluminescence quenching, of (3.9 ± 0.5) nm with a corresponding photoluminescence efficiency (ηPL) of (25 ± 1)% and thin film purity of (97.1 ± 1.2)%, measured by high performance liquid chromatography. After purification by thermal gradient sublimation, the value of LD is increased to (4.7 ± 0.5) nm with a corresponding ηPL of (33 ± 1)% and purity of (98.3 ± 0.8)%. Interestingly, a similar behavior is also observed as a function of the deposition boat temperature. Films deposited from the purified material at a high temperature give LD = (5.3 ± 0.8) nm with ηPL = (37 ± 1)% for films with a purity of (99.0 ± 0.3)% purity. Using a model of diffusion by Förster energy transfer, the variation of LD with purity is predicted as a function of ηPL and is in good agreement with measurements. The removal of impurities acts to decrease the non-radiative exciton decay rate and increase the radiative decay rate, leading to increases in both the diffusivity and exciton lifetime. The results of this work highlight the role of impurities in determining LD, while also providing insight into the degree of materials purification necessary to achieve optimized exciton transport.

An ice‐rich flow origin for the banded terrain in the Hellas basin, Mars

AbstractThe interior of Hellas Basin displays a complex landscape and a variety of geomorphological domains. One of these domains, the enigmatic banded terrain covers much of the northwestern part of the basin. We use high‐resolution (Context Camera and High‐Resolution Imaging Science Experiment) Digital Terrain Models to show that most of the complex viscous flowing behavior exhibited by the banded terrain is controlled by topography and flow‐like interactions between neighboring banded terrain. Furthermore, the interior of the basin hosts several landforms suggestive of the presence of near‐surface ice, which include polygonal patterns with elongated pits, scalloped depressions, isolated mounds, and collapse structures. We suggest that thermal contraction cracking and sublimation of near‐surface ice are responsible for the formation and the development of most of the ice‐related landforms documented in Hellas. The relatively pristine form, lack of superposed craters, and strong association with the banded terrain, suggest an Amazonian (&lt;3 Ga) age of formation for these landforms. Finally, relatively high surface pressures (above the triple point of water) expected in Hellas and summertime temperatures often exceeding the melting point of water ice suggest that the basin may have recorded relatively “temperate” climatic conditions compared to other places on Mars. Therefore, the potentially ice‐rich banded terrain may have deformed with lower viscosity and stresses compared to other locations on Mars, which may account for its unique morphology.

ON ORIGIN AND DESTRUCTION OF RELATIVISTIC DUST AND ITS IMPLICATION FOR ULTRAHIGH ENERGY COSMIC RAYS

Dust grains may be accelerated to relativistic speeds by radiation pressure, diffusive shocks, and other acceleration mechanisms. Such relativistic grains have been suggested as primary particles of ultrahigh energy cosmic rays (UHECRs). In this paper, we first revisit the problem of acceleration by radiation pressure and calculate maximum grain velocities achieved. We find that grains can be accelerated to relativistic speeds with Lorentz factor $\gamma < 2$ by powerful radiation sources, which is lower than earlier estimates showing that $\gamma$ could reach $\sim 10$. We then investigate different destruction mechanisms for relativistic grains traversing a variety of environments. In solar radiation, we find that the destruction by thermal sublimation and Coulomb explosions is important. We also quantify grain destruction due to electronic sputtering by ions and grain-grain collisions. Electronic sputtering is found to be rather inefficient, whereas the evaporation following grain-grain collisions is shown to be an important mechanism for which the $a \sim 0.01- 1\mu$m grains would be destroyed after sweeping a gas column $N_{Coul}\sim 5\times 10^{19}-5\times 10^{20}cm^{-2}$. Relativistic dust in the interstellar medium and intergalactic medium (IGM) would be disrupted by Coulomb explosions due to collisional charging after traversing a gas column $N_{Coul} \sim 10^{17}cm^{-2}$ unless grain material is very strong. We show that photoelectric emission by optical and ultraviolet background radiation is also significant for the destruction of relativistic dust in the IGM. The obtained results indicate that relativistic dust from galaxies would be destroyed before reaching the Earth's atmosphere and unlikely to account for UHECRs.

Mechanism for the separation of organic semiconductors via thermal gradient sublimation

Two common hole transporting materials, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) are separated and purified using thermal gradient sublimation. Their sublimation rates are proportional to the tube diameter squared and inversely proportional to the distance from the source boat to the deposition region. The deposition of these materials exponentially decays with distance down the tube. A theory predicting these results is derived. This theory suggests that diffusion and surface capture are the dominant steps in this purification, not processes in the source boat. It provides a rationale for improving the separation between multiple components and suggests routes for producing larger quantities of these materials.

Hydrogen desorption from hydrogen fluoride and remote hydrogen plasma cleaned silicon carbide (0001) surfaces

Due to the extreme chemical inertness of silicon carbide (SiC), in-situ thermal desorption is commonly utilized as a means to remove surface contamination prior to initiating critical semiconductor processing steps such as epitaxy, gate dielectric formation, and contact metallization. In-situ thermal desorption and silicon sublimation has also recently become a popular method for epitaxial growth of mono and few layer graphene. Accordingly, numerous thermal desorption experiments of various processed silicon carbide surfaces have been performed, but have ignored the presence of hydrogen, which is ubiquitous throughout semiconductor processing. In this regard, the authors have performed a combined temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) investigation of the desorption of molecular hydrogen (H2) and various other oxygen, carbon, and fluorine related species from ex-situ aqueous hydrogen fluoride (HF) and in-situ remote hydrogen plasma cleaned 6H-SiC (0001) surfaces. Using XPS, the authors observed that temperatures on the order of 700–1000 °C are needed to fully desorb C-H, C-O and Si-O species from these surfaces. However, using TPD, the authors observed H2 desorption at both lower temperatures (200–550 °C) as well as higher temperatures (&amp;gt;700 °C). The low temperature H2 desorption was deconvoluted into multiple desorption states that, based on similarities to H2 desorption from Si (111), were attributed to silicon mono, di, and trihydride surface species as well as hydrogen trapped by subsurface defects, steps, or dopants. The higher temperature H2 desorption was similarly attributed to H2 evolved from surface O-H groups at ∼750 °C as well as the liberation of H2 during Si-O desorption at temperatures &amp;gt;800 °C. These results indicate that while ex-situ aqueous HF processed 6H-SiC (0001) surfaces annealed at &amp;lt;700 °C remain terminated by some surface C–O and Si–O bonding, they may still exhibit significant chemical reactivity due to the creation of surface dangling bonds resulting from H2 desorption from previously undetected silicon hydride and surface hydroxide species.

Morphology and optical constants of nanographite films created by thermal vacuum deposition

The method for nanographite films preparation using thermal sublimation in vacuum of graphite nanoparticles under ultrasound treatment was proposed. Studied in the system nanographite film - substrate were composition and morphology of a multi- layer interface system by using ellipsometry and Raman scattering. It has been ascertained that the optical parameters of discontinuous nanographite films strongly depend on morphology of the surface layer. Using the results of ellipsometric measurements, we determined the effective optical constants, thickness of the films and calculated the volume composition of the solid phase in the film within the framework of the model of effective environment by Bruggeman.

Nanostructured explosives produced by vapor deposition: Structure and explosive properties

This paper generalizes the experimental data of the authors on the production and properties of thin-layer nanostructured explosives obtained by thermal vacuum sublimation. The method involves sublimation of explosive under heating in high vacuum, followed by deposition (condensation) of the explosive vapor on the substrate. Under these conditions, it has been shown that nanostructured polycrystalline layers of explosives containing a large number of micro-defects (pores and dislocations) are formed. In the explosive transformation in the deposited explosive layer, nano- and submicron-sized defects of the structure act as hot spots. The result is a significant reduction in the critical detonation dimensions. The nanostructured explosives studied by the authors are able to detonate at a layer thickness of 20–100 µm. Furthermore, their detonation velocity is substantially less dependent on the layer thickness than that of charges of the same explosives made by traditional technologies. Nanostructured explosives can also be used as components of explosive compositions with improved detonability.

Magnetic bistability in a submonolayer of sublimated Fe4 single-molecule magnets.

We demonstrate that Fe4 molecules can be deposited on gold by thermal sublimation in ultra-high vacuum with retention of single molecule magnet behavior. A magnetic hysteresis comparable to that found in bulk samples is indeed observed when a submonolayer film is studied by X-ray magnetic circular dichroism. Scanning tunneling microscopy evidences that Fe4 molecules are assembled in a two-dimensional lattice with short-range hexagonal order and coexist with a smaller contaminant. The presence of intact Fe4 molecules and the retention of their bistable magnetic behavior on the gold surface are supported by density functional theory calculations.

A Sublimation-driven Exospheric Model of Ceres

After Vesta, the NASA Dawn spacecraft will visit the dwarf planet Ceres to carry out in-depth observations of its surface morphology and mineralogical composition in 2015. One of the important questions is whether Ceres has any outgassing activity that would lead to the formation of a thin atmosphere. The recent detection of water vapor emitted from localized source regions by Herschel (Küppers et al., 2014) has only underscored this point. If the localized outgassing activity observed by Herschel is totally switched off, could a sizable surface-bounded exosphere still be maintained by other source mechanisms? Our preliminary assessment is that chemical sputtering via solar wind interaction and meteoroid impact are probably not adequate because of the large injection speed of the gas at production relative to the surface escape velocity of Ceres. One potential source is a low-level outgassing effect from its subsurface due to thermal sublimation with a production rate of the order of 1024 molecules s−1 as first considered by Fanale and Salvail (1989). If the water plumes are active, the fall-back of some of the water vapor onto Ceres’ surface would provide an additional global source of water molecules on the surface with a production rate of about 1025 molecules s−1. In this work, different scenarios of building up a tenuous exosphere by ballistic transport and the eventual recycling of the water molecules to the polar cold trap are described. It turns out that a large fraction of the exospheric water could be transferred to the polar caps area as originally envisaged for the lunar polar ice storage.

THE SENSING OF HUMIDITY BY SURFACE-TYPE Ag/FORMYL-TIPPCu(II)/Ag SENSOR FOR ENVIRONMENTAL MONITORING

In this paper, we have studied the effects of changing relative humidity on the electrical parameters and their multi frequency response of the surface type Ag /formyl- TIPPCu ( II )/ Ag humidity sensors. The silver electrodes of thickness 100 nm were primarily deposited on cleaned glass substrates by thermal evaporator. A gap of 40 μm was created between the electrodes of each device by using mask during the evaporation process. Thin films of formyl- TIPPCu ( II ) of 140 nm thickness were grown on silver electrodes by thermal sublimation technique. The values of capacitance and resistance of the sensors were found at different humidity levels at frequency of 1, 10 and 100 kHz of AC input signal. A remarkable increase in capacitance and decrease in resistance were observed during the rise of relative humidity from 45% to 95% RH. The hysteresis response of these humidity sensors was also studied at 1 kHz AC signal.

STRUCTURAL INTEGRITY OF SINGLE BIS(PHTHALOCYANINATO)-NEODYMIUM(III) MOLECULES ON METAL SURFACES WITH DIFFERENT REACTIVITY

Magnetic molecules are auspicious candidates to act as functional units in molecular spintronics. Integrating molecules into a device environment providing mechanical support and electrical contacts requires their deposition as intact entities onto substrates. Thermal sublimation is a very clean deposition process that, however, thermally decomposes molecules of insufficient stability leading to the deposition of molecular fragments. Here, we show that the molecule-surface interaction of chemisorbed molecules affects the intramolecular bonding and can lead depending on the surface reactivity to either molecular decomposition or enhanced stability. We study the integrity of single bis(phthalocyaninato)-neodymium(III) molecules ( NdPc 2) deposited by sublimation on differently reactive surfaces, namely Au (111), Cu (100), and two atomic layers of Fe on W (110), on the single molecular level by scanning tunneling microscopy (STM) and spectroscopy. We find a strongly substrate-dependent tendency of the NdPc 2 molecules to decompose into two Pc molecules. Surprisingly, the most reactive Fe / W (110) surface shows the lowest molecular decomposition probability, whereas there are no intact NdPc 2 molecules at all on the least reactive Au (111) surface. We attribute these findings to substrate-dependent partial charge transfer from the substrate to the Pc ligands of the molecule, which strengthens the intramolecular bonding mediated predominantly by electrostatic interaction.