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Papers by Gianluca Grimaldi

Proceedings of the nanoGe Spring Meeting 2022, 2022

Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being e... more Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being extensively explored for both photovoltaics and optoelectronic applications. The ionic nature of these materials make them ideal candidates for solution processing into both thin films and nanostructured crystals. However, a complete mechanistic description of 2D perovskite crystallization in solution is still missing due to the intricacy of process parameters and intermediates. Here, we investigate the role of different solid lead precursors (PbO 2 ,PbI 2 ,PbCO 3) on the crystallization of pure-phase, n=1, Ruddleson-Popper 2D perovskite BA 2 PbI 4 , during a two-step drop-cast-based synthesis. While BA 2 PbI 4 is formed in all cases, the nucleation and resulting morphology are strongly dependent on the choice of precursor, where the three lead precursors differ from each other in terms of their Pb-ion oxidation state, crystal structure and material class. We use in-situ optical live imaging during synthesis to reveal clear differences in crystallization kinetics of the same 2D perovskite as a function of the lead precursor. We discern three competing mechanisms in the Pb-precursor for the formation of BA 2 PbI 4 : dissolution/complexation, BAI intercalation and solid-state conversion. The differences in the oxidation state and solubility of the starting lead precursor in halide-rich solution play a key role in defining the crystallization pathway(s). This work demonstrates the importance of lead precursors in defining the nucleation and growth of perovskites thereby advancing the existing solution-processing techniques. Understanding how 2D perovskite crystals form in solution is key towards full control over their growth and opto-electronic properties, which will enable new types of physical phenomena and devices.

Proceedings of the nanoGe Spring Meeting 2022, 2022

ACS Applied Energy Materials, 2021

The Journal of Physical Chemistry Letters, 2021

The Journal of Physical Chemistry Letters, 2021

Ion migration in perovskite layers can significantly reduce the long-term stability of the device... more Ion migration in perovskite layers can significantly reduce the long-term stability of the devices. While perovskite composition engineering has proven an interesting tool to mitigate ion migration, many optoelectronic devices require a specific bandgap and thus require a specific perovskite composition. Here, we look at the effect of grain size to mitigate ion migration. We find that in MAPbBr 3 solar cells prepared with grain sizes varying from 2 to 11 μm the activation energy for bromide ion migration increases from 0.17 to 0.28 eV. Moreover, we observe the appearance of a second bromide ion migration pathway for the devices with largest grain size, which we attribute to ion migration mediated by the bulk of the perovskite, as opposed to ion migration mediated by the grain boundaries. Together, these results suggest the beneficial nature of grain engineering for reduction of ion migration in perovskite solar cells.

Light: Science & Applications, 2021

2D materials are considered for applications that require strong light-matter interaction because... more 2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton ce...

Advanced Materials Interfaces, 2020

To improve the stability and carrier mobility of quantum dot (QD) optoelectronic devices, encapsu... more To improve the stability and carrier mobility of quantum dot (QD) optoelectronic devices, encapsulation or pore infilling processes are advantageous. Atomic layer deposition (ALD) is an ideal technique to infill and overcoat QD films, as it provides excellent control over film growth at the sub‐nanometer scale and results in conformal coatings with mild processing conditions. Different thicknesses of crystalline ZnO films deposited on InP QD films are studied with spectrophotometry and time‐resolved microwave conductivity measurements. High carrier mobilities of 4 cm2 (V s)−1 and charge separation between the QDs and ZnO are observed. Furthermore, the results confirm that the stability of QD thin films is strongly improved when the inorganic ALD coating is applied. Finally, proof‐of‐concept photovoltaic devices of InP QD films are demonstrated with an ALD‐grown ZnO electron extraction layer.

The Journal of Physical Chemistry C, 2019

Chemistry of Materials, 2019

ACS Applied Energy Materials, 2018

Carrier multiplication (CM) is the process in which multiple electron−hole pairs are created upon... more Carrier multiplication (CM) is the process in which multiple electron−hole pairs are created upon absorption of a single photon in a semiconductor. CM by an initially hot charge carrier occurs in competition with cooling by phonon emission, with the respective rates determining the CM efficiency. Up until now, CM rates have only been calculated theoretically. We show for the first time how to extract a distinct CM rate constant from experimental data of the relaxation time of hot charge carriers and the yield of CM. We illustrate this method for PbSe quantum dots. Additionally, we provide a simplified method using an estimated energy loss rate to estimate the CM rate constant just above the onset of CM, when detailed experimental data of the relaxation time is missing.

Nanoscale, Jan 14, 2018

Indium antimonide (InSb) quantum dots (QDs) have unique and interesting photophysical properties,... more Indium antimonide (InSb) quantum dots (QDs) have unique and interesting photophysical properties, but widespread experimentation with InSb QDs is lacking due to the difficulty in synthesizing this material. The key experimental challenge in fabricating InSb QDs is preparing a suitable Sb-precursor in the correct oxidation state that reacts with the In-precursor in a controllable manner. Here, we review and discuss the synthetic strategies for making colloidal InSb QDs and present a new reaction scheme yielding small (∼1 nm diameter) InSb QDs. This was accomplished by employing Sb(NMe2)3 as the antimony precursor and by screening different reducing agents that can selectively reduce it to stibine in situ. The released SbH3, subsequently, reacts with In carboxylate to form small InSb clusters. The absorption features are moderately tunable (from 400 nm to 660 nm) by the amount and rate of reductant addition as well as the temperature of injection and subsequent annealing. Optical prop...

Journal of the American Chemical Society, 2018

Chemistry of Materials, 2018

ACS Applied Energy Materials, 2018

Proceedings of the nanoGe Spring Meeting 2022, 2022

Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being e... more Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being extensively explored for both photovoltaics and optoelectronic applications. The ionic nature of these materials make them ideal candidates for solution processing into both thin films and nanostructured crystals. However, a complete mechanistic description of 2D perovskite crystallization in solution is still missing due to the intricacy of process parameters and intermediates. Here, we investigate the role of different solid lead precursors (PbO 2 ,PbI 2 ,PbCO 3) on the crystallization of pure-phase, n=1, Ruddleson-Popper 2D perovskite BA 2 PbI 4 , during a two-step drop-cast-based synthesis. While BA 2 PbI 4 is formed in all cases, the nucleation and resulting morphology are strongly dependent on the choice of precursor, where the three lead precursors differ from each other in terms of their Pb-ion oxidation state, crystal structure and material class. We use in-situ optical live imaging during synthesis to reveal clear differences in crystallization kinetics of the same 2D perovskite as a function of the lead precursor. We discern three competing mechanisms in the Pb-precursor for the formation of BA 2 PbI 4 : dissolution/complexation, BAI intercalation and solid-state conversion. The differences in the oxidation state and solubility of the starting lead precursor in halide-rich solution play a key role in defining the crystallization pathway(s). This work demonstrates the importance of lead precursors in defining the nucleation and growth of perovskites thereby advancing the existing solution-processing techniques. Understanding how 2D perovskite crystals form in solution is key towards full control over their growth and opto-electronic properties, which will enable new types of physical phenomena and devices.

Proceedings of the nanoGe Spring Meeting 2022, 2022

ACS Applied Energy Materials, 2021

The Journal of Physical Chemistry Letters, 2021

The Journal of Physical Chemistry Letters, 2021

Ion migration in perovskite layers can significantly reduce the long-term stability of the device... more Ion migration in perovskite layers can significantly reduce the long-term stability of the devices. While perovskite composition engineering has proven an interesting tool to mitigate ion migration, many optoelectronic devices require a specific bandgap and thus require a specific perovskite composition. Here, we look at the effect of grain size to mitigate ion migration. We find that in MAPbBr 3 solar cells prepared with grain sizes varying from 2 to 11 μm the activation energy for bromide ion migration increases from 0.17 to 0.28 eV. Moreover, we observe the appearance of a second bromide ion migration pathway for the devices with largest grain size, which we attribute to ion migration mediated by the bulk of the perovskite, as opposed to ion migration mediated by the grain boundaries. Together, these results suggest the beneficial nature of grain engineering for reduction of ion migration in perovskite solar cells.

Light: Science & Applications, 2021

2D materials are considered for applications that require strong light-matter interaction because... more 2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton ce...

Advanced Materials Interfaces, 2020

To improve the stability and carrier mobility of quantum dot (QD) optoelectronic devices, encapsu... more To improve the stability and carrier mobility of quantum dot (QD) optoelectronic devices, encapsulation or pore infilling processes are advantageous. Atomic layer deposition (ALD) is an ideal technique to infill and overcoat QD films, as it provides excellent control over film growth at the sub‐nanometer scale and results in conformal coatings with mild processing conditions. Different thicknesses of crystalline ZnO films deposited on InP QD films are studied with spectrophotometry and time‐resolved microwave conductivity measurements. High carrier mobilities of 4 cm2 (V s)−1 and charge separation between the QDs and ZnO are observed. Furthermore, the results confirm that the stability of QD thin films is strongly improved when the inorganic ALD coating is applied. Finally, proof‐of‐concept photovoltaic devices of InP QD films are demonstrated with an ALD‐grown ZnO electron extraction layer.

The Journal of Physical Chemistry C, 2019

Chemistry of Materials, 2019

ACS Applied Energy Materials, 2018

Carrier multiplication (CM) is the process in which multiple electron−hole pairs are created upon... more Carrier multiplication (CM) is the process in which multiple electron−hole pairs are created upon absorption of a single photon in a semiconductor. CM by an initially hot charge carrier occurs in competition with cooling by phonon emission, with the respective rates determining the CM efficiency. Up until now, CM rates have only been calculated theoretically. We show for the first time how to extract a distinct CM rate constant from experimental data of the relaxation time of hot charge carriers and the yield of CM. We illustrate this method for PbSe quantum dots. Additionally, we provide a simplified method using an estimated energy loss rate to estimate the CM rate constant just above the onset of CM, when detailed experimental data of the relaxation time is missing.

Nanoscale, Jan 14, 2018

Indium antimonide (InSb) quantum dots (QDs) have unique and interesting photophysical properties,... more Indium antimonide (InSb) quantum dots (QDs) have unique and interesting photophysical properties, but widespread experimentation with InSb QDs is lacking due to the difficulty in synthesizing this material. The key experimental challenge in fabricating InSb QDs is preparing a suitable Sb-precursor in the correct oxidation state that reacts with the In-precursor in a controllable manner. Here, we review and discuss the synthetic strategies for making colloidal InSb QDs and present a new reaction scheme yielding small (∼1 nm diameter) InSb QDs. This was accomplished by employing Sb(NMe2)3 as the antimony precursor and by screening different reducing agents that can selectively reduce it to stibine in situ. The released SbH3, subsequently, reacts with In carboxylate to form small InSb clusters. The absorption features are moderately tunable (from 400 nm to 660 nm) by the amount and rate of reductant addition as well as the temperature of injection and subsequent annealing. Optical prop...

Journal of the American Chemical Society, 2018

Chemistry of Materials, 2018

ACS Applied Energy Materials, 2018