Picosecond ultrasonics is a powerful tool for nanoscale metrology, giving access to dimensions an... more Picosecond ultrasonics is a powerful tool for nanoscale metrology, giving access to dimensions and mechanical, thermal, and optical properties of nanomaterials. By monitoring the temporal evolution of the interaction of light with coherent acoustic phonons, also known as Brillouin oscillations, phonon lifetime and optical absorption can be measured. However, the extraction of these quantities can be inaccurate due to the common assumption of the infinite coherence length of probe pulses. Here, we demonstrate the effect of probe pulse duration on picosecond ultrasonic measurements numerically and experimentally. We establish a model that shows how the probe coherence length affects the measured signal loss and how we can overcome this limitation and measure an upper limit of the acoustic attenuation factor. The model is verified experimentally on a GaAs bulk substrate by varying the probe pulse duration, showing a strong effect for sub-100 fs pulses. Finally, we applied to CH3NH3PbBr3, where we reveal a high acoustic attenuation factor, which is in line with recent claims of strong anharmonicity in halide perovskites.
The spin polarization in centrosymmetric molybdenum disulfide (MoS 2) bulk crystal is recorded by... more The spin polarization in centrosymmetric molybdenum disulfide (MoS 2) bulk crystal is recorded by helicity dependent photocurrent exposed as elliptically polarized terahertz (THz) emission. We assign this observation to hidden spin polarization due to the local Dresselhaus effect induced by the molybdenum atomic site asymmetry. This spin polarization induces ultrafast optically controlled circular photogalvanic current that results in elliptically polarized THz emission. Furthermore, this THz radiation exhibits a helicity dependent bleaching effect under opposite circularly polarized excitations due to the spin relaxation. Our results demonstrate the viability of all-optical control of hidden spin polarization in two-dimensional materials and propose the applicability of the THz emission spectroscopy as a sensitive and contactless method to study spintronic physics in two-dimensional materials.
Physica Status Solidi B-basic Solid State Physics, Oct 16, 2020
Nanowire (NW) arrays containing a top segment of GaxIn1–xP are investigated, comparing NWs grown ... more Nanowire (NW) arrays containing a top segment of GaxIn1–xP are investigated, comparing NWs grown using two different Ga precursors, trimethylgallium (TMGa) and triethylgallium (TEGa). TMGa is the precursor commonly used for the particle‐assisted vapor–liquid–solid (VLS) growth of GaxIn1–xP NWs. However, it shows inefficient pyrolysis at typical NW growth conditions. The use of the alternative precursor TEGa is investigated by making a direct comparison between NWs grown using TEGa and TMGa at otherwise identical growth conditions. Growth rates, resulting NW materials composition, and time‐resolved photoluminescence (TRPL) lifetimes are investigated. With increasing Ga content of the NWs, the TRPL lifetimes decrease, indicating trap states that are associated with GaP. Somewhat longer TRPL lifetimes for the samples grown using TEGa indicate a lower concentration of deep trap states. For doped NWs, it is found that the strong effect of the p‐type dopant diethylzinc (DEZn) on the NW composition, observed for GaxIn1–xP NWs grown using TMGa, is absent when using TEGa.
Metal-passivated PbS nanoparticles promising to improve carrier transport in quantum-dot solar ce... more Metal-passivated PbS nanoparticles promising to improve carrier transport in quantum-dot solar cells are produced and are characterized layer-by-layer.
As one of the next-generation photovoltaic technologies, polymer solar cells (PSCs) possess the p... more As one of the next-generation photovoltaic technologies, polymer solar cells (PSCs) possess the potential to realize largearea solar panels with light weight, low cost, and mechanical In very recent years, growing efforts have been devoted to the development of all-polymer solar cells (all-PSCs). One of the advantages of all-PSCs over the fullerene-based PSCs is the versatile design of both donor and acceptor polymers which allows the optimization of energy levels to maximize the open-circuit voltage (V oc). However, there is no successful example of all-PSCs with both high V oc over 1 V and high power conversion efficiency (PCE) up to 8% reported so far. In this work, a combination of a donor polymer poly[4,8-bis(5-(2-octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6diyl-alt-(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)-1,3-diyl] (PBDTS-TPD) with a low-lying highest occupied molecular orbital level and an acceptor polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-thiophene-2,5-diyl] (PNDI-T) with a high-lying lowest unoccupied molecular orbital level is used, realizing high-performance all-PSCs with simultaneously high V oc of 1.1 V and high PCE of 8.0%, and surpassing the performance of the corresponding PC 71 BM-based PSCs. The PBDTS-TPD:PNDI-T all-PSCs achieve a maximum internal quantum efficiency of 95% at 450 nm, which reveals that almost all the absorbed photons can be converted into free charges and collected by electrodes. This work demonstrates the advantages of all-PSCs by incorporating proper donor and acceptor polymers to boost both V oc and PCEs.
still far behind those of inorganic counterparts, such as sc-Si, GaAs, CdTe, CIGS, and Perovskite... more still far behind those of inorganic counterparts, such as sc-Si, GaAs, CdTe, CIGS, and Perovskite solar cells. [2] One of the main reasons is their low open-circuit voltage (V oc) due to the large energy loss per absorbed photon. The minimum energy loss (E loss) is defined by the equation: E loss = E g − qV oc , where E g is the optical gap of the main light absorber, in most cases the donor material. Decreasing E loss will enhance the V oc and thus PCEs of PSCs. [3-5] The minimum E loss for efficient charge generation in organic solar cells (OSCs) is suggested to be 0.6 eV, [6] and E loss below 0.6 eV often leads to inefficient charge generation and low quantum efficiency, limiting the photocurrent and PCEs. [7] Typically, the E loss for polymer:fullerene based PSCs with high PCEs of 9%−11% is between 0.7 and 0.9 eV. [8-10] In contrast, Perovskite solar cells have a high V oc (> 1 V) with E loss of ≈0.5 eV, which partially accounts for their much higher PCEs of ≈20%. [11,12] The E loss values for inorganic solar cells, such as sc-Si, GaAs, GaInP, CdTe, CIGS-based solar cells are normally between 0.3 and 0.6 eV. [13] In Figure 1 and Table S1 in the Supporting Information, we summarize a selection of solar cells exhibiting either One of the factors limiting the performance of organic solar cells (OSCs) is their large energy losses (E loss) in the conversion from photons to electrons, typically believed to be around 0.6 eV and often higher than those of inorganic solar cells. In this work, a novel low band gap polymer PIDTT-TID with a optical gap of 1.49 eV is synthesized and used as the donor combined with PC 71 BM in solar cells. These solar cells attain a good power conversion efficiency of 6.7% with a high open-circuit voltage of 1.0 V, leading to the E loss as low as 0.49 eV. A systematic study indicates that the driving force in this donor and acceptor system is sufficient for charge generation with the low E loss. This work pushes the minimal E loss of OSCs down to 0.49 eV, approaching the values of some inorganic and hybrid solar cells. It indicates the potential for further enhancement of the performance of OSCs by improving their V oc since the E loss can be minimized.
We analyzed ultrafast photocarrier generation in single crystal CH<sub>3</sub>NH<s... more We analyzed ultrafast photocarrier generation in single crystal CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite through time-resolved terahertz spectroscopy. We find a Drude-Lorentz type conductivity spectra with a carrier mobility of 35 cm<sup>2</sup>/Vs and a Lorentzian component at 45 meV possibly due to intra-excitonic transitions.
Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measur... more Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measured at various excitation intensities. The average number of excitons per quantum dot was calculated from a Poisson model, which together with kinetic parameters was used to determine exciton population kinetics. Exciton and multiexciton absorption cross sections were determined and analyzed in terms of the electronic states of the quantum dots.
The subwavelength confinement and enhanced electric field created by plasmons allow precise sensi... more The subwavelength confinement and enhanced electric field created by plasmons allow precise sensing and enhanced light−matter interaction. However, the high frequency and short lifetime of plasmons limit the full potential of this technology. It is crucial to find substitutes and to study their dynamics. Here, we propose an experimental approach allowing the time-domain study of surface phonon polaritons. We first build a theoretical framework for the interaction of ultrashort pulses of light with polar materials. We then perform femtosecond pump− probe experiments and demonstrate the generation and timeresolved detection of surface phonon polaritons. By comparing experiments and simulations, we show the presence of both bright and dark modes with quality factors up to 115. We then investigate mode-dependent decay and energy transfer to the environment. Our results offer a platform for the experimental exploration of the dynamics of surface phonon polaritons and of the role of coherence in energy transfer.
Journal of Biomolecular Structure & Dynamics, Dec 1, 1985
Conformations of the synthetic deoxyoligonucleotide 17 base pairs long, which is an OR3 operator ... more Conformations of the synthetic deoxyoligonucleotide 17 base pairs long, which is an OR3 operator of λ phage, and of its 9-b.p. fragment were studied by the circular dichroism method (CD). The regions of stability of the double-stranded state were determined for these duplexes. A comparison of the CD spectra for these oligonucleotides with the CD for a lengthy DNA showed the conformation of these short DNA pieces to belong to the B-family.A cooperative change in the CD spectra is observed in trifluoroethanol (TFE) solutions at a TFE concentration specific for each oligonucleotide, which is supposed to stem from a B to A transition. The length of the fragment was found to affect the ability for the B-A transition. The transition into the A form is hindered by 13% TFE for the short 9-nucleotide in comparison with the 17-nucleotide. We suggest that this is due to the B form stabilization by terminal base pairs (B-phility of the ends).
Charge separation dynamics in a Ladder-Type Methyl substituted Poly(Para-Phenylene) (m-LPPP) was ... more Charge separation dynamics in a Ladder-Type Methyl substituted Poly(Para-Phenylene) (m-LPPP) was investgiated by means of electric filed modulated femtosecond pump-probe absorption spectroscopy. The Stark shift of the absorption band modified by created excitons and electron-hole pairs provided information about the average intrapair charge separation distance. The electron and hole initially separated by ca. 7 Å moved away from each other to more than 30 Å during 600 ps and later the separation rate decreased. Neither the initial separation distance, nor that obtained at 600 ps were found to depend on the applied electric field, only the separation rate at stronger field was slightly faster.
Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and L... more Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C6F5)3 resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two FeII cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the FeII-based t2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C6F5)3 attachment at the cyanido N lone pair.
Picosecond ultrasonics is a powerful tool for nanoscale metrology, giving access to dimensions an... more Picosecond ultrasonics is a powerful tool for nanoscale metrology, giving access to dimensions and mechanical, thermal, and optical properties of nanomaterials. By monitoring the temporal evolution of the interaction of light with coherent acoustic phonons, also known as Brillouin oscillations, phonon lifetime and optical absorption can be measured. However, the extraction of these quantities can be inaccurate due to the common assumption of the infinite coherence length of probe pulses. Here, we demonstrate the effect of probe pulse duration on picosecond ultrasonic measurements numerically and experimentally. We establish a model that shows how the probe coherence length affects the measured signal loss and how we can overcome this limitation and measure an upper limit of the acoustic attenuation factor. The model is verified experimentally on a GaAs bulk substrate by varying the probe pulse duration, showing a strong effect for sub-100 fs pulses. Finally, we applied to CH3NH3PbBr3, where we reveal a high acoustic attenuation factor, which is in line with recent claims of strong anharmonicity in halide perovskites.
The spin polarization in centrosymmetric molybdenum disulfide (MoS 2) bulk crystal is recorded by... more The spin polarization in centrosymmetric molybdenum disulfide (MoS 2) bulk crystal is recorded by helicity dependent photocurrent exposed as elliptically polarized terahertz (THz) emission. We assign this observation to hidden spin polarization due to the local Dresselhaus effect induced by the molybdenum atomic site asymmetry. This spin polarization induces ultrafast optically controlled circular photogalvanic current that results in elliptically polarized THz emission. Furthermore, this THz radiation exhibits a helicity dependent bleaching effect under opposite circularly polarized excitations due to the spin relaxation. Our results demonstrate the viability of all-optical control of hidden spin polarization in two-dimensional materials and propose the applicability of the THz emission spectroscopy as a sensitive and contactless method to study spintronic physics in two-dimensional materials.
Physica Status Solidi B-basic Solid State Physics, Oct 16, 2020
Nanowire (NW) arrays containing a top segment of GaxIn1–xP are investigated, comparing NWs grown ... more Nanowire (NW) arrays containing a top segment of GaxIn1–xP are investigated, comparing NWs grown using two different Ga precursors, trimethylgallium (TMGa) and triethylgallium (TEGa). TMGa is the precursor commonly used for the particle‐assisted vapor–liquid–solid (VLS) growth of GaxIn1–xP NWs. However, it shows inefficient pyrolysis at typical NW growth conditions. The use of the alternative precursor TEGa is investigated by making a direct comparison between NWs grown using TEGa and TMGa at otherwise identical growth conditions. Growth rates, resulting NW materials composition, and time‐resolved photoluminescence (TRPL) lifetimes are investigated. With increasing Ga content of the NWs, the TRPL lifetimes decrease, indicating trap states that are associated with GaP. Somewhat longer TRPL lifetimes for the samples grown using TEGa indicate a lower concentration of deep trap states. For doped NWs, it is found that the strong effect of the p‐type dopant diethylzinc (DEZn) on the NW composition, observed for GaxIn1–xP NWs grown using TMGa, is absent when using TEGa.
Metal-passivated PbS nanoparticles promising to improve carrier transport in quantum-dot solar ce... more Metal-passivated PbS nanoparticles promising to improve carrier transport in quantum-dot solar cells are produced and are characterized layer-by-layer.
As one of the next-generation photovoltaic technologies, polymer solar cells (PSCs) possess the p... more As one of the next-generation photovoltaic technologies, polymer solar cells (PSCs) possess the potential to realize largearea solar panels with light weight, low cost, and mechanical In very recent years, growing efforts have been devoted to the development of all-polymer solar cells (all-PSCs). One of the advantages of all-PSCs over the fullerene-based PSCs is the versatile design of both donor and acceptor polymers which allows the optimization of energy levels to maximize the open-circuit voltage (V oc). However, there is no successful example of all-PSCs with both high V oc over 1 V and high power conversion efficiency (PCE) up to 8% reported so far. In this work, a combination of a donor polymer poly[4,8-bis(5-(2-octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6diyl-alt-(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)-1,3-diyl] (PBDTS-TPD) with a low-lying highest occupied molecular orbital level and an acceptor polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-thiophene-2,5-diyl] (PNDI-T) with a high-lying lowest unoccupied molecular orbital level is used, realizing high-performance all-PSCs with simultaneously high V oc of 1.1 V and high PCE of 8.0%, and surpassing the performance of the corresponding PC 71 BM-based PSCs. The PBDTS-TPD:PNDI-T all-PSCs achieve a maximum internal quantum efficiency of 95% at 450 nm, which reveals that almost all the absorbed photons can be converted into free charges and collected by electrodes. This work demonstrates the advantages of all-PSCs by incorporating proper donor and acceptor polymers to boost both V oc and PCEs.
still far behind those of inorganic counterparts, such as sc-Si, GaAs, CdTe, CIGS, and Perovskite... more still far behind those of inorganic counterparts, such as sc-Si, GaAs, CdTe, CIGS, and Perovskite solar cells. [2] One of the main reasons is their low open-circuit voltage (V oc) due to the large energy loss per absorbed photon. The minimum energy loss (E loss) is defined by the equation: E loss = E g − qV oc , where E g is the optical gap of the main light absorber, in most cases the donor material. Decreasing E loss will enhance the V oc and thus PCEs of PSCs. [3-5] The minimum E loss for efficient charge generation in organic solar cells (OSCs) is suggested to be 0.6 eV, [6] and E loss below 0.6 eV often leads to inefficient charge generation and low quantum efficiency, limiting the photocurrent and PCEs. [7] Typically, the E loss for polymer:fullerene based PSCs with high PCEs of 9%−11% is between 0.7 and 0.9 eV. [8-10] In contrast, Perovskite solar cells have a high V oc (> 1 V) with E loss of ≈0.5 eV, which partially accounts for their much higher PCEs of ≈20%. [11,12] The E loss values for inorganic solar cells, such as sc-Si, GaAs, GaInP, CdTe, CIGS-based solar cells are normally between 0.3 and 0.6 eV. [13] In Figure 1 and Table S1 in the Supporting Information, we summarize a selection of solar cells exhibiting either One of the factors limiting the performance of organic solar cells (OSCs) is their large energy losses (E loss) in the conversion from photons to electrons, typically believed to be around 0.6 eV and often higher than those of inorganic solar cells. In this work, a novel low band gap polymer PIDTT-TID with a optical gap of 1.49 eV is synthesized and used as the donor combined with PC 71 BM in solar cells. These solar cells attain a good power conversion efficiency of 6.7% with a high open-circuit voltage of 1.0 V, leading to the E loss as low as 0.49 eV. A systematic study indicates that the driving force in this donor and acceptor system is sufficient for charge generation with the low E loss. This work pushes the minimal E loss of OSCs down to 0.49 eV, approaching the values of some inorganic and hybrid solar cells. It indicates the potential for further enhancement of the performance of OSCs by improving their V oc since the E loss can be minimized.
We analyzed ultrafast photocarrier generation in single crystal CH<sub>3</sub>NH<s... more We analyzed ultrafast photocarrier generation in single crystal CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite through time-resolved terahertz spectroscopy. We find a Drude-Lorentz type conductivity spectra with a carrier mobility of 35 cm<sup>2</sup>/Vs and a Lorentzian component at 45 meV possibly due to intra-excitonic transitions.
Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measur... more Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measured at various excitation intensities. The average number of excitons per quantum dot was calculated from a Poisson model, which together with kinetic parameters was used to determine exciton population kinetics. Exciton and multiexciton absorption cross sections were determined and analyzed in terms of the electronic states of the quantum dots.
The subwavelength confinement and enhanced electric field created by plasmons allow precise sensi... more The subwavelength confinement and enhanced electric field created by plasmons allow precise sensing and enhanced light−matter interaction. However, the high frequency and short lifetime of plasmons limit the full potential of this technology. It is crucial to find substitutes and to study their dynamics. Here, we propose an experimental approach allowing the time-domain study of surface phonon polaritons. We first build a theoretical framework for the interaction of ultrashort pulses of light with polar materials. We then perform femtosecond pump− probe experiments and demonstrate the generation and timeresolved detection of surface phonon polaritons. By comparing experiments and simulations, we show the presence of both bright and dark modes with quality factors up to 115. We then investigate mode-dependent decay and energy transfer to the environment. Our results offer a platform for the experimental exploration of the dynamics of surface phonon polaritons and of the role of coherence in energy transfer.
Journal of Biomolecular Structure & Dynamics, Dec 1, 1985
Conformations of the synthetic deoxyoligonucleotide 17 base pairs long, which is an OR3 operator ... more Conformations of the synthetic deoxyoligonucleotide 17 base pairs long, which is an OR3 operator of λ phage, and of its 9-b.p. fragment were studied by the circular dichroism method (CD). The regions of stability of the double-stranded state were determined for these duplexes. A comparison of the CD spectra for these oligonucleotides with the CD for a lengthy DNA showed the conformation of these short DNA pieces to belong to the B-family.A cooperative change in the CD spectra is observed in trifluoroethanol (TFE) solutions at a TFE concentration specific for each oligonucleotide, which is supposed to stem from a B to A transition. The length of the fragment was found to affect the ability for the B-A transition. The transition into the A form is hindered by 13% TFE for the short 9-nucleotide in comparison with the 17-nucleotide. We suggest that this is due to the B form stabilization by terminal base pairs (B-phility of the ends).
Charge separation dynamics in a Ladder-Type Methyl substituted Poly(Para-Phenylene) (m-LPPP) was ... more Charge separation dynamics in a Ladder-Type Methyl substituted Poly(Para-Phenylene) (m-LPPP) was investgiated by means of electric filed modulated femtosecond pump-probe absorption spectroscopy. The Stark shift of the absorption band modified by created excitons and electron-hole pairs provided information about the average intrapair charge separation distance. The electron and hole initially separated by ca. 7 Å moved away from each other to more than 30 Å during 600 ps and later the separation rate decreased. Neither the initial separation distance, nor that obtained at 600 ps were found to depend on the applied electric field, only the separation rate at stronger field was slightly faster.
Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and L... more Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C6F5)3 resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two FeII cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the FeII-based t2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C6F5)3 attachment at the cyanido N lone pair.
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Papers by Arkady Yartsev