In my research, I investigate charge, exciton and lattice dynamics using ultrafast spectroscopy in nanomaterials. For this, I employ broad range of ultrafast probes such as femtosecond lasers, THz waves, pulsed X-rays and electron beams.

My research spans a broad class of materials relevant to energy applications including colloidal nanocrystals, two-dimensional van der Waals heterostructures, metal-halide perovskites, IV-VI semiconductors and ferroelectric c oxides.

My goal is to uncover new functionalities in nanomaterials for energy-efficient devices in optoelectronics, photovoltaics and thermoelectrics.

Research highlights

Electron - lattice coupling in organic - inorganic hybrid perovskites

Ultrafast charge transfer in two-dimensional TMDC heterostructures

Anharmonic phonon dynamics in group IV-VI thermoelectric materials

Origin of photovoltaic response in perovskite-oxide ferroelectric materials

Optical gain and lasing in CdSe nanoplatelets

Nonradiative energy transfer in colloidal nanocrystals

Probing initial steps of charge separation and carrier - lattice coupling in organic-inorganic lead-halide perovskites

In recent years, hybrid lead-halide perovskites achieved record-breaking power conversion efficiencies (>22%) in solar cells. This has been enabled by combination of long carrier lifetimes, moderately high carrier mobilities and high defect tolerance in these materials. However, these properties had never existed all together in any other solution-processed semiconductor before. This led to scientific debates on the nature of the charge carriers and their separation in hybrid perovskites.

We have demonstrated that hybrid perovskites emit terahertz (THz) radiation immediately following photo-excitation. The emitted fields encode essential information related to initial charge separation and carrier - lattice coupling in these materials helping to uncover the origins of their surprising properties.

We found that initial charge separation is dominated by diffusion of carriers, not by previously-claimed ferroelectricity nor built-in surface fields. Moreover, we estimated individual carrier mobilities since our technique is directly sensitive to charge separation.

Moreover, we could time-resolve the excitation of longitudinal optical (LO) phonons by the electron motion, i.e., polaron formation dynamics.

Selected Publications:

  • B. Guzelturk, R. A. Belisle, M. D. Smith, K. Bruening, R. Prasanna, Y. Yuan, V. Gopalan, C. J. Tassone, H. I. Karunadasa, M. D. McGehee, A. M. Lindenberg,"Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling," Advanced Materials (2018).
  • X. Wu, L. Z. Tan, X. Shen, T. Hu, K. Miyata, M. Tuan Trinh, R. Li, R. Coffee, S. Liu, D. A. Egger, I. Makasyuk, Q. Zheng, A. Fry, J. S. Robinson, M. D. Smith, B. Guzelturk, H. I. Karunadasa, X. Wang, X.-Y. Zhu, L. Kronik, A. M. Rappe, A. M. Lindenberg, “Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites” Science Advances 3, e1602388 (2017). Link

Probing ultrafast charge transfer in 2D heterostructures of transition-metal dichalcogenides (TMDCs)

Two-dimensional materials field started by initial research on graphene (also awarded by Nobel prize in 2010). Now, this field is extending to other 2D materials which can provide different electronic functionality such as semiconductors, insulators, ferroelectrics, etc. From this respect, transition-metal dichalcogenide (TMDC) class (e.g., MoS2, WS2, WTe2 and etc.) offers broad range of tunability of these physical properties. Furthermore, hetero-structrues of TMDC 2D materials enable novel optoelectronic properties that led to world's thinnest opto-electronic devices such as photo-detectors and solar cells that are only three to four atoms thick.

For designing and developing these devices, it is essential to understand the motion of electrons in these heterostructures. Recently, we demonstrated that 2D heterostructures with Type-II band alignment leads to strong terahertz (THz) radiation analyzing which we can characterize the time-scale of the electron transfer from one layer to another layer in a bilayered system, forming a p-n junction at the atomic scale.

Our measurements enable us for the first time a direct access to the absolute efficiency of this charge transfer process. Within the first several hundered femtosecond following photo-excitation of the heterostructure, at least 70% of the excited state electrons migrate to the layer with lower conduction band energy. This efficient charge separation occuring at ultrafast timescale highlights the potential of these materials for efficient light-detection and photovoltaics systems.

Selected Publications:

  • E. Y. Ma*, B. Guzelturk*, G. Li, L. Cao, Z. X. Shen, A. M. Lindenberg, T. F. Heinz, “Recording Interfacial Currents on the Sub-Nanometer Length and Femtosecond Time Scale by Terahertz Emission” Science Advances Accepted for publication (2018). * Equal contribution

Optical gain in colloidal nanoplatelets and solution-processed nanocrystal lasers

Atomically-flat colloidal nanoplatelets show superior optical properties as compared to those of conventional colloidal quantum dots. My research has shown that nanoplatelets are exceptional candidates for optical gain and lasing. We showed low threshold lasing, long gain lifetimes and exceptionally large modal gain coefficients in these materials. These enabled all-solution-processed nanocrystal lasers.

Selected Publications:

  • B. Guzelturk, Y. Kelestemur, M. Olutas, S. Delikanli, H. V. Demir, “Amplified Spontaneous Emission and Lasing in Colloidal Nanoplatelets,” ACS Nano 8, 6599 (2014). Link
  • B. Guzelturk, Y. Kelestemur, M. Olutas, Q. Li, T. Lian, H. V. Demir, "High-Efficiency Optical Gain in Type-II Semiconductor Nanocrystals of Alloyed Colloidal Quantum Wells," J. Phys. Chem. Lett. 8, 5317 (2017). Link
  • B. Guzelturk, Y. Kelestemur, K. Gungor, A. Yeltik, M. Z. Akgul, W. Yue, R. Chen, H. D. Sun, C. Dang, H. V. Demir, “Stable and Low Threshold Optical Gain in CdSe/CdS Quantum Dots: All-Colloidal Frequency Up-Converted Laser" Adv. Mater. 27, 2741 (2015). Selected as Inside Front Cover. Link
  • B. Guzelturk, A. L. Kanibolotsky, C. Orofino-Pena, N. Laurand, M. D. Dawson, P. J. Skabara, H. V. Demir, “Ultralow-Threshold Up-Converted Lasing in Oligofluorenes with Tailored Strong Nonlinear Absorption,” J. Mater. Chem. C 3, 12018 (2015). Selected as Front Cover. Link
  • B. Guzelturk, Y. Kelestemur, M. Z. Akgul, V. K. Sharma, H. V. Demir, “Ultralow Threshold One-Photon and Two-Photon Pumped Optical Gain Media of Blue- Emitting Colloidal Quantum Dot Films,” J. Phys. Chem. Lett. 5, 2214 (2014). Link

Excitonic properties in colloidal II-VI nanoplatelet heterostructures

Colloidal nanoplatelets show promise for light-harvesting in solar cells and photo-catalysis thanks to their large absorption cross-section and strong light-matter interactions. We've demonstrated high efficiency exciton transfer reaching 99.9% within self-assembled nanoplatelet structures.

Selected Publications:

  • B. Guzelturk, O. Erdem, M. Olutas, Y. Kelestemur, H. V. Demir, “Stacking in Colloidal Nanoplatelets: Excitonic Properties,” ACS Nano 8, 12524 (2014). Link
  • M. Olutas*, B. Guzelturk*, Y. Kelestemur, A. Yeltik, S. Delikanli, H. V. Demir, “Lateral Size-Dependent Spontaneous and Stimulated Emission Properties in Colloidal CdSe Nanoplatelets” ACS Nano 9, 5041 (2015). * Equal contribution. Link
  • B. Guzelturk, M. Olutas, S. Delikanli, Y. Kelestemur, O. Erdem, H. V. Demir, “Nonradiative energy transfer in colloidal CdSe nanoplatelet films,” Nanoscale 7, 2345 (2015). Link
  • S. Delikanli, B. Guzelturk, P. L. Hernandez-Martinez, T. Erdem, Y. Kelestemur, M. Olutas, M. Z. Akgul, H. V. Demir, “Continuously Tunable Emission in Inverted Type-I CdS/CdSe Core/Crown Semiconductor Nanoplatelets,” Adv. Funct. Mater. 25, 4282 (2015). Link
  • B. Guzelturk, F. Menk, K. Philipps, Y. Kelestemur, M. Olutas, R. Zentel, H. V. Demir, “Colloidal Nanoplatelet/Conducting Polymer Hybrids: Excitonic and Material Properties,” J. Phys. Chem. C 120, 3573 (2016).

Exploiting nonradiative energy transfer using colloidal semiconductor nanocrystals

Selected Publications:

  • B. Guzelturk, H. V. Demir, “Near-Field Energy Transfer Using Nanoemitters For Optoelectronics,” Adv. Funct. Mater. 26, 8158 – 8177 (2016). Invited Feature Article. Link
  • B. Guzelturk, P. L. H. Martinez, Q. Zhang, Q. Xiong, H. Sun, X. W. Sun, A. O. Govorov and H. V. Demir, "Excitonics of semiconductor quantum dots and wires for lighting and displays," Laser & Photon. Rev. 8, 73 (2014). Selected as Front Cover. Link
  • B. Guzelturk, P. L. H. Martinez, V.K. Sharma, Y. Coskun, V. Ibrahimova, D. Tuncel, A. O. Govorov, X. W. Sun, Q. Xiong, H. V. Demir, “Study of exciton transfer in dense quantum dot nanocomposites,” Nanoscale 6, 11387 (2014). Link
  • E. Mutlugun*, B Guzelturk*, A. P. Abiyasa*, Y. Gao, X. W. Sun, H. V. Demir, “Colloidal Quantum Dot Light-Emitting Diodes Employing Phosphorescent Small Organic Molecules as Efficient Exciton Harvesters,” J. Phys. Chem. Lett. 5, 2802 (2014). * Equal contribution. Link
  • B. Guzelturk, P. L. H. Martinez, D. Zhao, X. W. Sun, H. V. Demir, “Singlet and Triplet Exciton Harvesting in the Thin Films of Colloidal Quantum Dots Interfacing Phosphorescent Small Organic Molecules,” J. Phys. Chem. C 118, 25964 (2014). Link
  • S. Nizamoglu, B. Guzelturk, D.-W. Jeon, I. -H. Lee, H. V. Demir, “Efficient exciton migration from InGaN/GaN nanopillars to CdSe/ZnS core/shell nanocrystals,” Appl. Phys. Lett. 98, 163108 (2011). Link
  • M. Olutas, B. Guzelturk, Y. Kelestemur, K. Gungor, H. V. Demir, “Highly Efficient Nonradiative Energy Transfer from Colloidal Semiconductor Quantum Dots to Wells for Sensitive Non-contact Temperature Probing” Adv. Funct. Mater. 26, 3570 (2016). Link
  • B. Guzelturk, H. V. Demir, “Organic-Inorganic Composites of Semiconductor Nanocrystals For Efficient Excitonics” J. Phys. Chem. Lett. 6, 2206 (2015). Invited Perspective Letter, selected as Front Cover. Link
  • A.Yeltik*, B. Guzelturk*, P. L. H. Martinez, A. O. Govorov, H. V. Demir, “Phonon assisted nonradiative energy transfer into silicon from colloidal quantum dots,” ACS Nano 7, 10492 (2013).* Equal contribution. Link