CrystEngComm Blog

Posted on behalf of Josh Campbell, web writer for CrystEngComm 

Researchers in the field of nanophotonics aim to control photons or optical energies at the nanometre scale by using devices; typically 1D nanomaterials such as nanotubes or nanowires. Due to the nanoscopic nature of such materials, the quantum confinement effect allows fascinating properties to emerge which can be harnessed to produce ultra-fast, low-power and interference-free devices.

An integral part of any nanophotonic device is the waveguide, a physical structure that guides the photons to their target location. Waveguides can be active, guiding photons via coupling mechanisms, or passive, propagating the source light directly through the material.

Of all the classes of waveguide materials, inorganic devices are the most common, however recent research into waveguides made from small molecule organics is gaining traction. The advantages of these organic devices over existing inorganic materials are that they are easier to produce and have tuneable properties arising from greater variations in molecular structure.

For the emerging field of biophotonics, biocompatibility is a key requirement, with non-toxic pharmaceutical molecules being a logical fit for the role but not having been well explored.

In a recent article in CrystEngComm, researchers from the Rajadurai and Chandrasekar groups evaluated three pharmaceutical molecules as biophotonic devices: caffeine, carbamazepine and gilbenclamide, with nanoscale samples of each molecule grown via drop-casting on a clean glass slide.

The researchers found that crystal growth was governed by kinetic factors which often left the sample with defects which are compounded by defects created by the source of radiation used to examine the structures. To overcome this, the authors used a novel method of Raman laser light confinement to characterise the compounds.

All three molecules exhibited tubular morphologies which, in the case of carbamazepine, measured hundreds of microns in length. Passive waveguiding was observed via 2D optical confinement in all samples and no unnatural defects were observed when the samples were subjected to Raman spectroscopy.

The potential of these materials to act as biocompatible optical waveguides will hopefully bring the first biophotonic nano-devices significantly closer to realisation.

Read the full article now for more details: 

Passive optical waveguiding tubular pharmaceutical solids and Raman spectroscopy/mapping of nano-/micro-scale defects
Naisa Chandrasekhar, E. Ramanjaneya Reddy, Muvva D. Prasad, Marina S. Rajadurai and   Rajadurai Chandrasekar 
CrystEngComm, 2014, DOI: 10.1039/C4CE00084F

Josh Campbell is a PhD student, currently at the University of Southampton, UK studying crystal structure prediction of organic semiconductors. He received his BSc from the University of Bradford.

Nanometer- or micrometer-sized particles, doped with a small quantity of rare-earth cations, exhibit two types of luminescence. Where the light absorbed is of higher energy than the light emitted (known as down-conversion or DC) the materials can be used in lighting and displays.  If the light absorbed is of lower energy than the light emitted (up-conversion or UC), the materials can be used in photonics and biological imaging.  The luminescence behaviour depends on the composition, size and shape of the particles and the rare-earth ion (or ions) used for doping.

Lanthanide oxyfluorides, such as YOF, are attractive candidates for the host particles, due to their high stability and good transparency.  These materials have been prepared with various particle sizes but using harsh conditions and complicated processes which can, crucially, leave behind traces of the organic molecules used to control morphology.  These can be detrimental to the physical and chemical properties of the final product.

A new paper shows how a simple hydrothermal method can be used to prepare YOF particles with controllable size and shape, determined by altering the pH of the reaction mixture and without the need for organic shape-directing reagents.   At pH 9 microrods form, while at pH 11 the particles form as nanospheres and at pH 14 there is a mixture of the two morphologies.  The UV luminescence properties of samples doped with the rare-earth cations Tm³⁺, Tb³⁺ or Eu³⁺show characteristic blue, green or red DC emissions. Samples doped with two different rare earth cations, under lower energy excitation,  show red, blue and green UC emissions for Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Ho³⁺ doped particles, respectively (see diagram below).

The emission intensities are related to the particle size and the number of surface defects (which lead to quenching of the luminescence).  Intensities are therefore highest for the microrods which are largest and have fewest defects.  Authors conclude that the YOF particles prepared are excellent host lattices for efficient luminescence which could find application in colour displays and anti-counterfeit labels.

For more details see the paper at:

YOF nano/micro-crystals: morphology controlled hydrothermal synthesis and luminescence properties

Yang Zhang, Xuejiao Li, Dongling Geng, Mengmeng Shang, Hongzhou Lian, Ziyong Cheng and Jun Lin

CrystEngComm, 2014, Advance Article
DOI: 10.1039/C3CE42323A, Paper

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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. Currently, she is writing a book on chemicals from plants

Posted on behalf of Josh Campbell, web writer for CrystEngComm 

Graphene is a material composed of single of 2D sheets of graphite which showcases a number of exotic properties. These include: ballistic transport of charge, which occurs partially due to the material having a lower resistivity than that of silver; an anomalous quantum Hall effect and spin transport. Single crystals of graphene can be grown using chemical vapour deposition (CVD) on a variety of substrates although the material is perhaps more famously known for being prepared from graphite using adhesive tape in a process called exfoliation. These different methods of preparation influence the final properties of the material; with CVD-produced samples often having lower mobilities than exfoliated samples, which are smaller. Epitaxy, the process of growing one crystalline material on another with recognition of some form between the layers, is another viable method of graphene synthesis, with heteroepitaxal growth having been investigated for a variety of different substrates. In this vein, it has been postulated that growing “graphene-on-graphene” could offer methods for both investigating the mechanism of graphene growth and producing large single crystal samples. 

A recent article in CrystEngComm reports how homoepitaxal growth can proceed from both exfoliated and CVD grown samples of graphene. In the study, an exfoliated or CVD-grown seed was placed onto a copper foil surface and heated to 1025 °C in the presence of H₂ and CH₄. By investigating the atomic structure around the newly grown graphene, the authors showed that the crystal orientation was preserved from the original graphite flake and the graphene sheet, with graphene layers 1-2 sheets thick being made regardless of the method used. The authors subsequently used the result to grow large films of graphene epitaxally. A close examination of the atomic structures of both the seed and the newly grown graphene showed that the original crystal orientation was preserved during growth. It is hoped that this new method of homoepitaxal graphene growth will allow for much larger and higher-quality samples of crystalline material to be grown in the future. 

Read the full article now for more details: 

Lateral homoepitaxial growth of graphene
H. Wang, G. Wang, P. Bao, Z. Shao, X. Zhang, S. Yang, W. Zhu and   B. Deng
CrystEngComm, 2014, DOI: 10.1039/C3CE42072H

Josh Campbell is a PhD student, currently at the University of Southampton, UK studying crystal structure prediction of organic semiconductors. He received his BSc from the University of Bradford.

Successful use of pharmaceutical drugs depends on their delivery and controlled release so that their bioactivity can be harnessed.  This can mediate poor solubility, degradation and other properties of the drug which might otherwise be problematic.  One way to control delivery is to load the drug into a container which allows the compound to be transported to the desired location, to then be released over a suitable time period.  The behaviour of the container is dependent on both the size and the shape, so simple and reliable fabrication techniques are required.

In a recent CrystEngComm article, scientists from China show how such containers can be made which are shaped like lotus leaves and are nano/microsized.  The Co₃O₄ nano/microcontainers can be easily prepared from Co(NO₃)₂.6H₂O by evaporation of the acetone solvent followed by calcining (i.e. heating at below the melting point).   In this process, shown in the diagram below, the large amount of gas bubbles produced are key to determining the shape of the containers, with no other shape-directing agents required.  The size and density of the nano/microlotus-leaf arrays can be controlled by variation of the evaporation time and temperature.

The research team used fluorescein isothiocyanate (FITC) as a model drug to study the controlled drug delivery from the nano/microlotus-leaf arrays.  They found that it could be loaded and released more effectively than for comparable Co₃O₄ microspheres and showed that cells which were treated with the arrays retained over 80% viability even at high concentration — indicating that these microcontainers are a safe delivery vehicle of active compounds to cells.

For more details, see the paper:

Facile bubble-assisted evaporation-induced assembly of high-density arrays of Co₃O₄nano/microlotus leaves: fluorescent properties, drug delivery, and biocompatibility

 Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. Currently, she is writing a book on chemicals from plants. 

Posted on behalf of Josh Campbell, web writer for CrystEngComm 

The phenomenon of jumping crystals was first reported in 1983 when it was discovered that heating crystals of (phenylazophenyl)palladiumhexafluoroacetylacetonate caused not only a polymorphic transition, but also the sample to literally “jump” off the heat source.  Since then, mechanically responsive materials research has blossomed due to potential applications in medical devices, actuators and electronic sensors. Most research in the field has focussed on molecular scale movement (typically using rotaxanes or catenanes) or light-activated polymers for macroscopic movement. However, single crystals possess many of the properties needed for practical applications of actuators.  The ordered structure of the crystal should allow any induced effect to travel faster which allows faster energy transfer, shorter response times and faster recovery. 

A new highlight article in CrystEngComm recaps some of the recent progress in the field, focusing on the mechanical processes seen in thermo- and photo-responsive crystals. Thermally induced jumping is known as the thermosalient effect and has been reported for many materials. Crystals that exhibit this effect generally fall into three types: crystals which contain hydrogen bonds; crystals without hydrogen bonding groups that form stacked layers; and crystals with no strong intermolecular interactions whatsoever. For the second class, the ability of a layered structure to exhibit hopping depends on the interactions between layers and the thermal motion of the atoms between them. Oxitropium bromide is one such crystalline material, which undergoes a reversible conformational change which has the effect of loading, and then decompressing, a spring. For polar molecules, a compression along the most polar axis can lead to mechanical effects. 

Photo-responsive crystals rely on the photochromic effect, a reversible transformation that occurs on the absorption of electromagnetic radiation. Crystals can respond mechanically to this absorption with a range of mechanical effects. For example, microcrystals of trans-4-aminoazobenzene bend away from UV light. Bending and curling is often seen with cis–trans isomerism, ring opening and closure and cycloaddition reactions. The photosalient effect has also been reported, whereby mechanical strain develops in the crystal due to a photochemical reaction. An example of this is α-santonin, its crystals turn yellow and burst when exposed to sunlight. 

Research into thermal and photo induced mechanical effects is now picking up compared to previous decades after the realisation of their importance in both energy conversion and for developing mechanically responsive materials. 

Read the article now for more details: 

Thermally induced and photoinduced mechanical effects in molecular single crystals—a revival 

N. K. Nath, M. K. Panda, S. C. Sahooa and P. Naumov
CrystEngComm, 2014, DOI: 10.1039/C3CE41313F 

Josh Campbell is a PhD student, currently at the University of Southampton, UK studying crystal structure prediction of organic semiconductors. He received his BSc from the University of Bradford.