Tuning Phosphorous Allotropes for Optoelectronics

An optical image of black phosporus mid-IR photodetector.

The element phosphorus has several different allotropes, including the thermodynamically stable form, black phosphorus (BP). BP has interesting properties which make it useful for the optoelectrical field, such as its layered structure, bandgap in the mid-infrared range and high carrier mobility. 

HgxCd(1-x)Te (MCT) is generally regarded as the most popular mid-infrared material, whose composition can be tuned by in material growth process. However dynamical, in-situ tuning of its optical properties has never been achieved, limiting its ability. 

In this paper, researchers discovered black phosphorus could be useful for in-situ tunable mid-infrared applications. They leveraged a thin layer of black phosphorus sandwiched between hexagonal boron nitride (HBN) and applied an electric field to tune its optical properties. This expanded the photo-response of the mid-infrared photodetectors from 3.7 to 7.7 µm. Other than photodetectors, high speed mid-infrared modulators can be readily constructed using the same concept. 

They used the heating and cooling probe stage, the HFS600E-PB4, together with an FTIR spectrometer for the temperature-dependent photo-response measurements. 

Their results prove promising. The layered nature of BP, the high intrinsic mobility and strong photo-response in the broad mid-IR wavelength range make it an ideal material for high-speed mid-IR photodetectors, modulators and spectrometers. 
 

By Tabassum Mujtaba

Xia et al., Widely tunable black phosphorus mid-infrared photodetector. (2017) Nature Communicationsvolume 8, Article number: 1672 doi:10.1038/s41467-017-01978-3

Studying Phase Transitions in Pharmaceuticals with the Linkam DSC450

Dr Asma Buanz

UCL School of Pharmacy, University College London, 29-39 Brunswick Square, London, WC1N 1AX

Background

Polymorphism in pharmaceutical solids has great implications on both the processing and the performance of solid pharmaceutical products. It is the ability of a substance to exist in more than one molecular arrangement and the result is more than one polymorphic forms which differ in their physiochemical properties such as solubility, stability, melting point etc.1 Depending on these arrangements the polymorphic forms could vary in their relative stabilities; with the metastable forms eventually converting to the most stable form.1,2. Studying these phase transformations is important in understanding the properties of these polymorphic forms. Various techniques could be employed for this purpose but Differential Scanning Colorimetry is the most common and efficient technique as it allows following these transformations as a function of temperature or time, in addition to its high sensitivity.3 Nonetheless, sometimes it is difficult to build a clear picture of what is happening to the sample as it goes through a phase transition from just the heat flow signal provided by the DSC, and thus visualising these processes would be valuable. In addition, subtle transitions such as solid-solid transitions could be missed in the DSC if they happen over a wide temperature range.

Method

The Linkam DSC450 stage allows visualisation of the sample during a DSC experiment. Therefore, this system was used in studying flufenamic acid, one of the most polymorphic pharmaceuticals with a record of nine known polymorphic forms2. The aim was to study crystallisation from the amorphous phase obtained by melt quenching. Form I was obtained by spray drying and was first heated in the DSC450  up to the melt, then it was allowed to cool down to room temperature before re-heating at a 10 °C/min heating rate.

Results

As shown in Figure 1, form I melted at ca. 132 °C while the re-heated sample melted at a lower temperature (onset of ca. 122 °C). No re-crystallisation was observed in the second heating cycle, which indicated that upon cooling a metastable form recrystallised from the melt. 

The effect of adding a polymer (PVP) is evident in Figure 2 where it appeared that the sample did not crystallise upon cooling but rather formed an amorphous phase. Heating the amorphous phase caused there-crystallisation of FFA followed by a solid-solid transition and then a melt. These events appear as two exothermic transitions followed by a sharp endotherm. The solid-solid transition is subtle in the DSC thermogram but is very clear from the signal obtained from employing an image analysis technique (Thermal Analysis by Surface Characterization, TASC) shown in Figure 2c. The melting peak has an onset temperature of ca. 119 °C, which is lower than that of the form crystallised from the melt without the presence of the polymer. The TASC signal also shows that melting is detected visually before the DSC signal starts to change.

Conclusion

In this work polymorphic transitions in the pharmaceutical active flufenamic acid were studied with Linkam DSC450 stage, which combines optical microscopy with differential scanning calorimetry. The power of the complementary technique was evident with the increased sensitivity for detecting subtle transitions such as solid-solid transition by analysing the optical images.

References

1. Rodrı́g uez-Spong, B., Price, C. P., Jayasankar, A., Matzger, A. J. and Rodrı́guez-Hornedo, N. r. 2004. General principles of pharmaceutical solid polymorphism: A supramolecular perspective. Advanced Drug Delivery Reviews 56(3): 241-274.

2. López-Mejías, V., Kampf, J. W. and Matzger, A. J. 2012. Nonamorphism in flufenamic acid and a new record for a polymorphic compound with solved structures. Journal of the American Chemical Society 134(24): 9872-9875.

3. Gaisford, S. and Saunders, M. 2012. Physical form i – crystalline materials. Essentials of pharmaceutical preformulation, John Wiley & Sons, Ltd: 127-155.

February's Paper of the Month

Nanomaterials have been found to have interesting electronic, magnetic and optical properties. They can manipulate electromagnetic fields through localised surface plasmon resonance to modulate light interactions. Such plasmonic phenomena are popular in application for the biomedical field. 

February’s Paper of the Month comes from the University of California, Merced and Stanford University. They developed a micro-scale delivery module for various organic and inorganic compounds using nanomaterials. 

Their aim was to create something that would be versatile and capable of encapsulating a range of different materials (drugs, dyes, cells, bacteria, etc.) for many different applications. These could include drug delivery for cancer treatment, releasing dyes in vivo for fluorescence imaging, or tissue engineering. The problem with most current platforms is that they are either leaky, unable to hold the contents without loss for any prolonged period, or they are incapable of releasing contents in a spatially and temporally controlled manner. For example, other cargo delivery systems that use light to activate the release of the cargo need several milliwatts of power over several minutes to achieve the required effect, therefore creating significant localised heating. The group managed to reduce the power required to less than 2 mW and the release time to under 5 seconds. As a result, the total temperature increase at the vicinity of the capsules is only to ~ 40°C, which is well within tolerable limits for many biological systems.

They used an LTS350* for their experiments. When asked on the importance of the hotstage, Dr Ghosh said: “One of the most critical parameters that determine whether a cargo delivery system is viable in vivo is the thermal gradient that is produced because of the photothermal effect when optical excitation used to rupture the shells is in resonance with the plasmonic response of the nanoparticles that make up the shell walls. To estimate this, the first step was to use heat to rupture the shells instead of light. That is where we used the heating stage.”
 

Fluorescence microscopy images of a Nano-Assembled Microshell loaded with a fuorescent dye on the LTS350 stage. 

 Their method has proved to be exciting and advantageous. No leakage was seen for over five months after encapsulation, promising a long shelf life. Furthermore, a lower optical intensity was required for shell disintegration compared to other methods. 

Although more work is required to improve future in-vivo applications (such as actuation by near infra-red and reducing overall size of capsule), their work is a promising result for future cargo delivery systems. 

By Tabassum Mujtaba

Ghosh et al., Plasmon-actuated nano-assembled microshells. Sci. Rep. 7. doi:10.1038/s41598-017-17691-6

*The LTS350 has been superseded by the LTS420 offering a large temperature range and better temperature control to 0.01°C.
 

January's Paper of the Month

The efficiency of lead halide perovskites has increased significantly since their introduction in 2009. The high performance as well as cost-effective manufacture make them ideal for photovoltaic applications. However, lead halide perovskites are known for their instability when subjected to changes in light, oxygen, temperature as well as issues with homogeneity. As such it becomes important to test their stability in a practical situation, such as in a device stack, to better understand the degradation mechanics. 

Raman spectroscopy is a powerful probing tool to study the degradation of individual perovskite layers as well as the degradation kinetics. Raman mapping techniques can also be used to investigate the homogeneity of the perovskite film. 

January’s Paper of the Month comes from the University of Swansea. They used a Linkam RH95 and THMS600-H to conduct in-situ Raman Spectroscopy and further understand the degradation kinetics of perovskite materials when temperature and humidity were altered. 

The device stack was first tested for thermal degradation by heating to 150°C. While photo-degradation was found to be dependent on top and bottom layers, thermal degradation was shown to be non-reversible and affects the entire MAPI film. It is determined by the homogeneity of the film rather than structural layers. Raman signals from in-situ humidity experiments show the dihydration of the perovskite to be almost completely reversible once drying occurs. However further analysis of the raman peaks showed dihyration remained in the Au region, suggesting some moisture remained trapped in this region. Device performance may be fully recovered if trapped moisture can be removed.
 

Figure 1: The in situ raman spectra highlights the reversible dihydration when relative humidity is decreased but also indicates the presence of a dehydrated species in the Au region. 

Understanding how humidity affects perovskite layers differentially within a stack has led to the conclusion that targeted drying would help improve and regain device performance. Optimising such performances will have great benefits for the future development of microelectronics and telecommunication.

By Tabassum Mujtaba

Tsoi et al., Probing the degradation and homogeneity of embedded perovskite semiconducting layers in photovoltaic devices by Raman spectroscopy. Phys.Chem.Chem.Phys. 19, 5246 (2017).