• Het Kranenveld 14, Helix (STO 0.33)

    5612 AE Eindhoven

    Netherlands

  • P.O. Box 513, Department of Chemical Engineering and Chemistry

    5600 MB Eindhoven

    Netherlands

Research Output 1981 2020

Article
10 Citations (Scopus)
1 Downloads (Pure)

Using lenses to improve the output of a patterned luminescent solar concentrator

Tsoi, S., Broer, D. J., Bastiaansen, C. W. M. & Debije, M. G., 2013, In : Advanced Energy Materials. 3, 3, p. 337-341

Research output: Contribution to journalArticleAcademicpeer-review

Solar concentrators
Lenses
Waveguides
Carbonates
Coloring Agents
12 Citations (Scopus)
58 Downloads (Pure)

Using magnetic birefringence to determine the molecular arrangement of supramolecular nanostructures

Gielen, J. C., Shklyarevskiy, I. O., Schenning, A. P. H. J., Christianen, P. C. M. & Maan, J. C., 2009, In : Science and Technology of Advanced Materials. 10, 1, p. 1-6 6 p.

Research output: Contribution to journalArticleAcademicpeer-review

Open Access
File
Birefringence
Nanostructures
Magnetic fields
Optical anisotropy
Magnetic susceptibility
35 Citations (Scopus)
14 Downloads (Pure)

Visual performance of red luminescent solar concentrating windows in an office environment

Vossen, F. M., Aarts, M. P. J. & Debije, M. G., 2016, In : Energy and Buildings. 113, p. 123-132

Research output: Contribution to journalArticleAcademicpeer-review

Solar concentrators
Solar energy
Energy efficiency
Innovation
Glass
14 Citations (Scopus)
52 Downloads (Pure)

Volume generation towards dynamic surface morphing in liquid crystal polymer networks

Liu, D., 7 Dec 2016, In : Liquid Crystals. 43, 13-15, p. 2136-2143 8 p.

Research output: Contribution to journalArticleAcademicpeer-review

Open Access
File
Liquid Crystals
Azobenzene
Liquid crystal polymers
Ultraviolet radiation
Liquid crystals
14 Citations (Scopus)
1 Downloads (Pure)

Water-responsive dual-coloured photonic polymer coatings based on cholesteric liquid crystals

Stumpel, J., Broer, D. & Schenning, A., 2015, In : RSC Advances. 5, 115, p. 94650-94653 4 p.

Research output: Contribution to journalArticleAcademicpeer-review

6 Citations (Scopus)
48 Downloads (Pure)

Well-adhering, easily producible photonic reflective coatings for plastic substrates

van Heeswijk, E. P. A., Kloos, J. J. H., de Heer, J., Hoeks, T., Grossiord, N. & Schenning, A. P. H. J., 5 Sep 2018, In : ACS Applied Materials & Interfaces. 10, 35, p. 30008-30013 6 p.

Research output: Contribution to journalArticleAcademicpeer-review

Open Access
File
Reflective coatings
Photonics
Plastics
Coatings
Substrates
49 Citations (Scopus)

X-ray structures and redox properties of Copper(II) Bis(pyrazole) complexes

Martens, C. F., Schenning, A. P. H. J., Feiters, M. C., Berens, H. W., van der Linden, J. G. M., Admiraal, G., Beurskens, P. T., Kooijman, H., Spek, A. L. & Nolte, R. J. M., 1 Sep 1995, In : Inorganic Chemistry. 34, 19, p. 4735-4744 10 p.

Research output: Contribution to journalArticleAcademicpeer-review

Amines
Copper
Nitrogen
X rays
copper
1 Citation (Scopus)

厚度依赖的聚合物稳定胆甾相液晶的电控红外反射器件

Translated title of the contribution: Cell thickness dependence of electrically tunable infrared reflectors based on polymer stabilized cholesteric liquid crystalsHu, X., de Haan, L. T., Khandelwal, H., Schenning, A. P. H. J., Nian, L. & Zhou, G., 1 May 2018, In : Science China. Materials. 61, 5, p. 745-751 7 p.

Research output: Contribution to journalArticleAcademicpeer-review

Open Access
Cholesteric liquid crystals
Polymers
Cells
Infrared radiation
Bandwidth
Book editing
28 Citations (Scopus)

Cross-linked liquid crystalline systems: from rigid polymer networks to elastomers

Broer, D. J. (ed.), Crawford, G. P. (ed.) & Žumer, S. (ed.), 1 Jan 2011, CRC Press. 615 p.

Research output: Book/ReportBook editingAcademicpeer-review

Elastomers
Liquid Crystals
elastomers
Liquid crystals
Polymers

Liquid crystal sensors

Schenning, A. P. H. J. (ed.), Crawford, G. P. (ed.) & Broer, D. J. (ed.), 2018, Boca Raton: CRC Press. 164 p. (The liquid crystals book series)

Research output: Book/ReportBook editingAcademicpeer-review

Responsive polymer surfaces: dynamics in surface topography

Liu, D. (ed.) & Broer, D. J. (ed.), 2017, Weinheim: Wiley-VCH Verlag. 257 p.

Research output: Book/ReportBook editingAcademicpeer-review

Surface topography
Polymers
Acrylates
Hydrogels
Silicones
Chapter
3 Downloads (Pure)

3-D molecular architectures in thin film liquid crystalline networks

Broer, D. J., Lub, J. & Mol, G. N., 1998, Chemical and physical networks. Nijenhuis, te, K. & Mijs, W. J. (eds.). New York: Wiley, p. 361-375 (The Wiley Polymer Networks Group review series; vol. 1).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademic

liquids
thin films
monomers
polarizers
visible spectrum

Chiral nematic liquid crystalline sensors containing responsive dopants

Cachelin, P. & Bastiaansen, C. W. M., 2018, Liquid crystal sensors. Schenning, A. P. H. J., Crawford, G. P. & Broer, D. J. (eds.). Boca Raton: CRC Press, p. 63-82 (The Liquid Crystals Book Series).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Liquid Crystals
Doping (additives)
Crystalline materials
Sensors
Liquids
2 Citations (Scopus)
2 Downloads (Pure)

Cholesteric liquid crystalline polymer networks as optical sensors

Moirangthem, M. & Schenning, A. P. H. J., 1 Jan 2017, Liquid Crystal Sensors. Schenning, A. P. H. J., Crawford, G. P. & Broer, D. J. (eds.). Boca Raton: CRC Press, p. 83-101 19 p. (The Liquid Crystals Book Series).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Liquid crystal polymers
Optical sensors
optical measuring instruments
Refractive index
refractivity
7 Citations (Scopus)

Densely cross-linked liquid crystal networks by controlled photopolymerization of ordered liquid crystal monomers: properties and applications

Lub, J. & Broer, D. J., 24 Jan 2011, Densely Cross-Linked Liquid Crystal Networks by Controlled Photopolymerization of Ordered Liquid Crystal Monomers: Properties and Applications. crawford, G., zumer, S. & broer, D. (eds.). Boca Raton: CRC Press, p. 4-47 46 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Liquid Crystals
Photopolymerization
Monomers
Polymers
Optical properties
4 Citations (Scopus)
1 Downloads (Pure)

Dye-doped polysiloxane rubbers for luminescent solar concentrator systems

Buffa, M. & Debije, M. G., 2014, High-Efficiency Solar Cells. Wang, X. & Wang, Z. M. (eds.). Berlin: Springer, p. 247-266 (Springer Series in Materials Science; vol. 190).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademic

Siloxanes
Solar concentrators
polycarbonate
Solar collectors
Rubber
1 Downloads (Pure)

Dynamic tribology in liquid crystal coatings

Liu, D. & Broer, D. J., 2017, Responsive polymer surfaces: dynamics in surface topography. Liu, D. & Broer, D. J. (eds.). Weinheim: Wiley-VCH Verlag, p. 123-135

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

tribology
friction
liquid crystals
coatings
static friction
111 Citations (Scopus)

Hydrogen-bonded assemblies of dyes and extended π-conjugated systems

Ajayaghosh, A., George, S. J. & Schenning, A. P. H. J., 15 Aug 2005, Supermolecular Dye Chemistry. Wuerthner, F. (ed.). Berlin: Springer, p. 83-118 36 p. (Topics in Current Chemistry; vol. 258).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Hydrogen
Coloring Agents
Azo Compounds
Porphyrins
Oligomers
4 Downloads (Pure)

Hydrogen bonding in supramolecular nanoporous materials

Kuringen, van, H. P. C. & Schenning, A. P. H. J., 2015, Hydrogen Bonded Supramolecular Materials. Li, Z-T. & Wu, L-Z. (eds.). Berlin: Springer, p. 43-67 25 p. (Lecture Notes in Chemistry).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademic

Hydrogen bonds
Hydrogen
Molecules
Liquid crystal polymers
Catalysis

Inkjet printed liquid crystal cilia

van Oosten, C. L., Bastiaansen, C. W. M. & Broer, D. J., 2017, Responsive polymer surfaces: dynamics in surface topography. Liu, D. & Broer, D. J. (eds.). Weinheim: Wiley-VCH Verlag, p. 35-56

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Liquid Crystals
Actuators
Monomers
Azo Compounds
Methacrylates
4 Downloads (Pure)

Light-​actuated artificial cilia based on liquid crystal networks

Oosten, van, C. L., Bastiaansen, C. W. M. & Broer, D. J., 2013, Artificial Cilia. Toonder, den, J. & Onck, P. (eds.). Cambridge: Royal Society of Chemistry, (RSC Nanoscience & Nanotechnology; vol. 30).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

liquid crystals
actuators
fabrication
fluidics
versatility
2 Downloads (Pure)

Liquid crystal coatings switched between flat and corrugated surface texture

Liu, D. & Broer, D. J., 2017, Responsive polymer surfaces: dynamics in surface topography. Liu, D. & Broer, D. J. (eds.). Weinheim: Wiley-VCH Verlag, p. 57-83

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

flat surfaces
textures
liquid crystals
coatings
topography
4 Citations (Scopus)
3 Downloads (Pure)

Luminescent solar concentrators

Chatten, A. J., Bose, R., Farrell, D. J., Xiao, Y., Chan, N. L. A., Manna, L., Büchtemann, A., Quilitz, J., Debije, M. G. & Barnham, K. W. J., 2010, Nanotechnology for Photovoltaics. Tsakalakos, L. (ed.). London: CRC Press, p. 323-349 27 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Solar concentrators
concentrators
Photovoltaic cells
photovoltaic cells
Multiplexing equipment

Multicomponent self-assembled polymers based on pi-conjugated systems

Kaeser, A. G., Wolffs, M. & Schenning, A. P. H. J., 2012, Soft matter. Steed, J.W., X. & Gale, Ph.A., X. (eds.). New York: Wiley, p. 3183-3205 4014 p. (Supramolecular chemistry : from molecules to nanomaterials; vol. 7).

Research output: Chapter in Book/Report/Conference proceedingChapterAcademic

4 Citations (Scopus)

Nanoparticles based on π-conjugated polymers and oligomers for optoelectronic, imaging, and sensing applications: the illustrative example of fluorene-based polymers and oligomers

Fischer, I. & Schenning, A. P. H. J., 2013, Organic Electronics : Emerging Concepts and Technologies. Cicoira, F. & Santato, C. (eds.). Wiley-VCH Verlag, p. 1-25

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

On the role of hydrogen-bonding in the nanoscale organization of π-conjugated materials

Schenning, A. P. H. J. & Gonzalez Rodriguez, D., 2013, Organic Nanomaterials : Synthesis, Characterization, and Device Applications. Torres, T. & Bottari, G. (eds.). p. 33-57 24 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Hydrogen bonds
Polymers
Functional materials
Amides
Urea

Photoresponsive polymer hydrogel coatings that change topography

Stumpel, J. E., ter Schiphorst, J. & Schenning, A. P. H. J., 2017, Responsive polymer surfaces: dynamics in surface topography. Liu, D. & Broer, D. J. (eds.). Weinheim: Wiley-VCH Verlag, p. 159-173

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Hydrogel
Topography
Wetting
Polymers
Microfluidics

Polymer MEMS

van Oosten, C. L., Bastiaansen, C. W. M. & Broer, D. J., 1 Jan 2011, Cross-Linked Liquid Crystalline Systems: From Rigid Polymer Networks to Elastomers. Broer, D., Crawford, G. P. & Zumer, S. (eds.). Boca Raton: CRC Press, p. 251-285 35 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

microelectromechanical systems
MEMS
Polymers
micrometers
polymers
2 Downloads (Pure)

Smart materials

Broer, D. J., Houten, van, H. E. H., Ouwerkerk, M., Toonder, den, J. M. J., Sluis, van der, P., Klink, S. I., Hikmet, R. A. M. & Balkenende, R., 2006, True visions : the emergence of ambient intelligence. Aarts, E. H. L. & Encarnacao, J. L. (eds.). Berlin: Springer, p. 53-82

Research output: Chapter in Book/Report/Conference proceedingChapterAcademic

1 Citation (Scopus)

Stereospecific a-olefin polymerization with heterogeneous catalysts

Severn, J. & Jones, R. L., 23 Apr 2018, Handbook of Transition Metal Polymerization Catalysts. Second ed. Wiley-Blackwell, p. 229-312 84 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Alkenes
Olefins
Polymerization
Catalysts
Polypropylenes
27 Downloads (Pure)

Stimuli-responsive liquid crystalline materials

Debije, M. G. & Schenning, A. P. H. J., 2016, Reference module in materials science and materials engineering. Hashmi, S. (ed.). Oxford: Elsevier, 9 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Liquid Crystals
Optical materials
Materials science
Nanostructured materials
Self assembly

Switching surface topographies based on liquid crystal network coatings

Liu, D. & Broer, D. J., 19 May 2017, Photomechanical Materials, Composites, and Systems: Wireless Transduction of Light into Work. New Jersey: Wiley-Liss Inc., p. 303-326 24 p.

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review

Liquid Crystals
Surface topography
Liquid crystals
topography
liquid crystals
Comment/Letter to the editor
1 Citation (Scopus)
11 Downloads (Pure)

Erratum: author correction: liquid crystal elastomer coatings with programmed response of surface profile (Nature communications (2018) 9 1 (456))

Babakhanova, G., Turiv, T., Guo, Y., Hendrikx, M., Wei, Q. H., Schenning, A. P. H. J., Broer, D. J. & Lavrentovich, O. D., 14 Mar 2018, In : Nature Communications. 9, 1, 1 p., 1130 .

Research output: Contribution to journalComment/Letter to the editorAcademicpeer-review

Open Access
File
document markup languages
Elastomers
Ficus
Liquid Crystals
HTML
Conference article

Holographic phase gratings in back- and frontlights for LCD's

Bastiaansen, C. W. M., Van Heesch, C. & Broer, D. J., 1 Dec 2006, In : Proceedings of International Meeting on Information Display. 2006, 1 p.

Research output: Contribution to journalConference articleAcademicpeer-review

Liquid crystal displays
Light polarization
Color
Polarization
Energy efficiency
6 Citations (Scopus)

Light-induced orientation of dyes in azobenzene containing LC polymers

Rosenhauer, R., Fischer, T., Stumpe, J., Gimenez, R., Pinol, M., Serrano, J. L. & Broer, D., 1 Dec 2002, In : Proceedings of SPIE - The International Society for Optical Engineering. 4799, p. 121-135 15 p.

Research output: Contribution to journalConference articleAcademicpeer-review

Azobenzene
Dyes
Polymers
Coloring Agents
dyes

Polarized back and frontlights for LCDs

Van Heesch, C., Jagt, H., Cornelissen, H., Broer, D. J. & Bastiaansen, C., 1 Dec 2004, In : Proceedings of SPIE - The International Society for Optical Engineering. 5289, p. 170-181 12 p.

Research output: Contribution to journalConference articleAcademicpeer-review

Liquid crystal displays
polarized light
Light polarization
waveguides
Waveguide
6 Citations (Scopus)

Polymeric helices with submicron dimensions for MEMS devices

Elias, A. L., Harris, K. D., Bastiaansen, C. W. M., Broer, D. J. & Brett, M. J., 9 Dec 2005, In : Proceedings of SPIE - The International Society for Optical Engineering. 5836, p. 41-55 15 p., 05.

Research output: Contribution to journalConference articleAcademicpeer-review

Liquid Crystalline Polymer
Liquid crystal polymers
Helix
Micro-electro-mechanical Systems
helices

The influence of laser writing of polyimides on the alignment of liquid crystals

Versteeg, D. J., Bastiaansen, C. W. M. & Broer, D. J., 1 Dec 2001, In : SID Conference Record of the International Display Research Conference. p. 65-68 4 p.

Research output: Contribution to journalConference articleAcademicpeer-review

Polyimides
Liquid crystals
Lasers
2 Citations (Scopus)

Use of lyotropic liquid crystals for patterned polarizer and retarder applications

Sousa, M. E., Crawford, G. P., Bastiaansen, C. W. M. & Broer, D. J., 1 Dec 2006, In : Digest of Technical Papers - SID International Symposium. 37, 2, p. 836-839 4 p.

Research output: Contribution to journalConference articleAcademicpeer-review

Optical devices
Liquid crystals
Display devices
Fluids
Conference contribution
4 Citations (Scopus)

3D-structured liquid crystal networks formed by dichroic photoinitiator initiated photopolymerization

Serrano-Ramón, B., Kjellander, C., Zakerhamidi, S., Bastiaansen, C. W. M. & Broer, D. J., 15 May 2008, Emerging Liquid Crystal Technologies III. 691109. (Proceedings of SPIE - The International Society for Optical Engineering; vol. 6911).

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Liquid Crystals
Photopolymerization
Liquid Crystal
Liquid crystals
monomers
1 Downloads (Pure)

A concept for switchable, energy-generating ‘smart’ windows

Debije, M. G., 2010, Proceedings of the 25th European Photovoltaic Solar Energy Conference. München, Germany: WIP, p. 5153-5156

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademic

energy
dyes
photovoltaic cells
electricity
glass
5 Citations (Scopus)
2 Downloads (Pure)

A low-power CMOS integrated sensor for CO2 detection in the percentage range

Humbert, A., Tuerlings, B. J., Hoofman, R. J. O. M., Tan, Z., Gravesteijn, D. J., Pertijs, M. A. P., Bastiaansen, C. W. M. & Soccol, D., 2013, 2013 Transducers and Eurosensors XXVII: The 17th International Conference on Solid-State Sensors, Actuators and Microsystems, TRANSDUCERS and EUROSENSORS 2013. p. 1649-1652 4 p. 6627101

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Atmospheric humidity
Humidity sensors
Sensors
Carbon dioxide
Capacitance
4 Citations (Scopus)
14 Downloads (Pure)

A new design for luminescent solar concentrating PV roof tiles

Doudart de la Gree, G. C. H., Papadopoulos, A., Debije, M. G., Cox, M. G. D. M., Krumer, Z., Reinders, A. H. M. E. & Rosemann, A. L. P., 2015, Proceedings of the 42nd Photovoltaic Specialists Conference, June 14-19 2015, New Orleans, USA.

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Tile
Roofs
Dyes
Ray tracing
Conceptual design
4 Citations (Scopus)

Angular dependence of surface-relief gratings for solar and lighting applications

de Jong, T. M., de Boer, D. K. G. & Bastiaansen, C. W. M., 17 Oct 2011, Nonimaging Optics: Efficient Design for Illumination and Solar Concentration VIII. Bellingham: SPIE, 8 p. 81240D. (Proceedings of SPIE - The International Society for Optical Engineering; vol. 8124).

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Diffraction efficiency
illuminating
Gratings
Lighting
Diffraction Efficiency
3 Downloads (Pure)

Anisotropic light emissions in luminescent solar concentrators

Verbunt, P. P. P. C., Sánchez-Somolinos, C., Broer, D. J. & Debije, M. G., 1 Dec 2012, Optics for Solar Energy, OSE 2012. p. ST2A.7 3 p.

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Solar concentrators
Light emission
concentrators
light emission
Dyes
1 Downloads (Pure)

Colourful Photo-Curable Coatings for Application in the Electro-Optical Industry

Lub, J., Broer, D. J. & Witte, van de, P., 2001, 27th Proceedings of the Athens Conference on Coatings: Science and Technology. Greece, Athens, p. 209-217

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Developing efficient upconverter silicon solar cell devices

Goldschmidt, J. C., Aeberhard, U., Capek, R., Debije, M. G., Evers, W., Fischer, S., Fröhlich, B., Hermle, M., Herter, B., Ivaturi, A., Krämer, K. W., Lifshitz, E., Martín-Rodríguez, R., MacDougall, S. K. W., Meijerink, A., Moulin, E. A., Müller, T. C. M., Paetzold, U., Richards, B. S., Wolf, S. & 3 others, Warzecha, M., Wouters, C. & Zhao, Y., 1 Jan 2013, Optical Nanostructures for Photovoltaics, PV 2013. Optical Society of America (OSA)

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

renewable energy
Silicon solar cells
Photonic crystals
Semiconductor quantum dots
Energy gap
7 Citations (Scopus)

Dynamical properties of mutually delayed coupled semiconductor lasers

Erzgräber, H., Lenstra, D., Krauskopf, B. & Fischer, I., 2004, Semiconductor lasers and laser dynamics, 27 - 30 April 2004, Strasbourg, France. Lenstra, D., Morthier, G., Erneux, T. & Pessa, M. (eds.). Bellingham, Washington: SPIE, p. 352-361 (Proceedings of SPIE; vol. 5452).

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

semiconductor lasers
lasers
laser modes
coupled modes
differential equations
2 Citations (Scopus)

Efficient and cost-effective polarized-light backlights for LCDs

Cornelissen, H. J., Jagt, H. J. B., Broer, D. J. & Bastiaansen, C. W. M., 12 Nov 2008, Eighth International Conference on Solid State Lighting. Bellingham: SPIE, 10 p. 70580X. (Proceedings of SPIE - The International Society for Optical Engineering; vol. 7058).

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Light polarization
Liquid crystal displays
polarized light
Metal foil
foils
1 Citation (Scopus)
8 Downloads (Pure)

Electric field switched surface topography of fingerprint liquid-crystal network polymer coating

Feng, W., Liu, D. & Broer, D. J., 2018, Emerging Liquid Crystal Technologies XIII. Chien, L-C. (ed.). SPIE, 105551B. (Proceedings of SPIE; vol. 10555).

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

Liquid Crystals
Surface Topography
Surface topography
Fingerprint
Liquid Crystal