Quantum Nature of Light in Non-Stoichiometric Bulk Perovskites | ACS

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Quantum Nature of Light in Non-Stoichiometric Bulk Perovskites Daniel G. Suarez-Forero, Antonella Giuri, Milena De Giorgi, Laura Polimeno, Luisa De Marco, Francesco Todisco, Giuseppe Gigli, Lorenzo Dominici, Dario Ballarini, Vincenzo Ardizzone, Benny D. Belviso, Davide Altamura, Cinzia Giannini, Rosaria Brescia, Silvia Colella, Andrea Listorti, Carola Esposito Corcione, Aurora Rizzo, and Daniele Sanvitto ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b05361 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Quantum Nature of Light in Non-Stoichiometric Bulk Perovskites †,‡

†,‡

Daniel G. Suárez-Forero,

†,¶

Polimeno,

Dominici,

†

†

Altamura,

∗,†,¶

Listorti,

†

Luisa De Marco,

Dario Ballarini,

§,#

∗,†

Antonella Giuri,

Milena De Giorgi,

Francesco Todisco,

†

†,¶

Vincenzo Ardizzone,

§

Cinzia Giannini,

Rosaria Brescia,

†,‡

Carola Esposito Corcione,

k

Giuseppe Gigli,

†,¶

Benny D. Belviso,

†,¶

Silvia Colella,

Aurora Rizzo,

†,¶

Laura

Lorenzo

§

Davide

Andrea

and Daniele Sanvitto

†,⊥

†CNR NANOTEC, Institute of Nanotechnology, Via Monteroni, 73100 Lecce, Italy ‡Dipartimento di Ingegneria dell'Innovazione, Università del Salento, via per Monteroni,

km 1, 73100 Lecce, Italy' ¶Dipartimento di Fisica, Universitá del Salento, Strada Provinciale Lecce-Monteroni,

Campus Ecotekne, Lecce 73100, Italy §Istituto di Cristallograa, CNR-IC, Via Amendola 122/O, 70126 Bari, Italy kElectron Microscopy Facility, Istituto Italiano di Tecnologia, via Morego 30, Genova, Italy ⊥INFN Sezione di Lecce, 73100 Lecce, Italy #Istituto di Cristallograa, CNR-IC, Via Amendola 122/O, 70126 Bari, Italy E-mail: [email protected]; [email protected]

Abstract Sources of single photons are a fundamental brick in the development of quantum information technologies. Great eorts have been made so far, in the realisation of reliable, highly ecient and on demand quantum sources that could show an easy 1

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integration with quantum devices. This has recently culminated in the use of solid state quantum dots as promising candidates for future sources of quantum technologies. However, some challenges, like their complex fabrication, random distribution and dicult integrability with silicon technology could hinder their broad application, making necessary the study of alternative systems. In this work, we clearly demonstrate the single photon emission from quantum dots formed in non stoichiometric bulk perovskites. Their simple growing procedures, exceptional stability under constant illumination, easy control of their optical properties, as well as ease of integrability, make these materials very interesting candidates for the development of quantum light sources in the near infrared.

Keywords perovskites, quantum dots, single photon emitters, stochiometry, correlation

Introduction The emergence of quantum technologies, in which the information is stored, processed, and communicated according to the laws of quantum physics, makes single photon sources one of the essential elements needed for feeding quantum circuits with non-classical light. Nowadays the most common single-photon sources are based on spontaneous parametric down conversion (SPDC) or on the emission from two level systems, which are inherently quantum in nature.

1

A critical issue in using frequency conversion in nonlinear crystals is that the

probability of creating

n

pairs scales with pump intensity to the n

th

power, what imposes

an upper limit to the number of qubits generated, making their processing very cumbersome, especially if non-linear eects want to be exploited.

2,3

For this reason single photon

emitters, despite several diculties on their isolation and/or reproducibility, represent one of the most promising systems for the generation of optical quantum bits.

2


In particular,

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semiconductor-based quantum dots (QDs),

4,5

due to their clear advantage in terms of scala-

bility are recently demonstrating exceptional performances. InGaAs QDs, for instance, have shown a high degree of indistinguishability, high quantum eciency and a narrow bandwidth,

6

with state-of-the-art repetition rate of up to 300 MHz.

these QDs, it has been proved that using active demultiplexing

7

More recently, by using it is possible to generate

six photons at a rate one order of magnitude higher than standard SPDC systems. However, III-V semiconductors work only at helium temperatures, are not very easy to integrate in silicon based technologies and are usually limited to a range of emission energies between 850 and 950 nm, leaving room for the development of QD systems based on dierent material platforms. These include defects in wide-bandgap inorganic semiconductors, organic molecules in host matrices, colloidal nanocrystals, nitrogen vacancies in diamonds and defects in transition metal dichalcogenides.

817

Very recently, an interesting class of materials, the hybrid perovskites described by the general formula ABX3 with A, B, and X being an organic or inorganic cation, a metal cation, and a halide anion, respectively have attracted a great attention due to their spectacular performances in photovoltaics, and emission tunability erties.

26,27

18,19

2325

their simple growing procedures,

2022

very strong absorption

, high quantum yields and extremely interesting physical prop-

In terms of quantum sources, it was demonstrated that full inorganic perovskites

CsPbX3 (X = I, Br) colloidal QDs can be used for the generation of single photons.

28,29

However, from a technological point of view, the use of colloidal QDs is not always the best choice since it requires complex chemical syntheses and, in most cases, it does not allow a ne control of the optical properties, nor an easy integration with optical circuits due to their fast degradation under constant illumination

28

and long decay times.

29

In this direction, the possibility to use bulk materials, and manipulate the properties of hybrid organic inorganic perovskites by simply changing their precursors, anticipates a wide range of applications in which the versatility provided by the easy optoelectronic control, and the integrability of the bulk structure, could be extensively exploited.

3


26,30

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In this work, we demonstrate near-infrared single photon emission from localized domains comprised in polycrystalline perovskite thin lms.

This is possible by an

ad-hoc,

control-

lable modication of the precursors stoichiometry, resulting in the formation of nanometric crystalline domains from which single photon emission takes place at low temperature. In particular, we report very narrow linewidths (down to antibunching

31

0.36

nm or

0.7

meV) and photon

under pulsed excitation, as clear evidence for the formation of single QDs.

Furthermore we also demonstrate that these two level systems embedded in the perovskite matrix do not show blinking nor signicant spectral diusion of their emission line for many hours of continuos excitation. This system represents an easy, low cost and scalable way for the development of innovative single photon sources.

Results and discussion Morphological and structural analysis:

Dierent formamidinium lead iodide perovskite

(FAPI) polycristalline lms were prepared and studied (see Methods). Using same precursors and solvent and changing stoichiometry and growth parameters such as annealing temperature and spin speed rate in lm preparation, it is shown (see Supporting information) that the stoichiometry is the main parameter inuencing the optical properties of the FAPI polycristalline lms. High-resolution transmission electron microscopy (HR-TEM) analyses have been carried out on the stochiometric and non-stoichiometric samples. Overview images show a rather dierent morphology in the two cases (Fig. 1). Whereas for FAPI 1:1 the material is homogeneous (Fig. 1a-b), in the case of 2:1 FAPI, isolated nanometer-sized crystalline domains are present (Fig. 1e-f ), whose dimensions are comparable with the Bohr radius of 6.35 nm

32

estimated by using the values of reduced eective mass (µ) and eective dielectric constant (ef f ) calculated by Galkowski

et al. 33

for bulk FAPI perovskites.

Analysis of HR-TEM

images, through their fast Fourier transform (FFT), revealed the crystalline nature of the

4


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Figure 1: (a) HRTEM image of the lm FAPI 1:1 with a corresponding zoomed portion in (b).

(c) FFT of (b), matching with

α-FAPI.

(e) HRTEM image of the lm FAPI 2:1

with a corresponding zoomed portion in (f ). The areas enclosed by the dashed yellow lines correspond to FAPI crystalline domains. (g) FFT of (f ), matching with of emissive centers in samples FAPI 1:1 (d) and FAPI 2:1 (h).

α-FAPI. PL spectra

In the non stoichiometric

case, it is possible to detect narrow peaks from spatially localized domains. Images of panels d) and h) are taken at

T =4

K

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lms: for stoichiometric FAPI, mostly domains in the the presence of some domains in the

δ

Page 6 of 21

α

FAPI phase are found Fig. 1c, with

phase (see panel c and Fig. S5 in the SI). In the case

of non-stoichiometric FAPI, instead, only domains in the

α phase are identied (see Fig. 1g).

This is in agreement with X-Ray Diraction (XRD) analysis (see Fig. S7 in the SI). Informa-

Figure 2: PL spectra of FAPI 2:1 for dierent temperatures in a range from 4 K to 50 K. The sample shows sharp emission from spatially localized bright spots at temperatures below 20 K.

tion about the elemental composition of the samples was extracted from energy dispersive x-ray spectroscopy (EDS) measurements revealing, in 2:1 perovskite, the presence of a higher amount of nitrogen (from formamidinium) and iodine, with respect to the lead amount (see

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supporting information Fig. S6). Our hypothesis is that the formation of nanometric FAPI islands in non-stoichiometric perovskite can be assisted by the not homogeneous distribution of organic cations (in excess with respect to lead iodide) that disrupts the continuity of the lm: amorphous formamidinium in excess could conne small crystalline domains.

Figure 3: Real space maps for dierent energies, taken with a wavelength window of 0.2 nm on the non stoichiometric (2:1) FAPI perovskite: a) 835.9 nm (1.483 eV), b) 838.5 nm (1.479 eV), c) 841.0 nm (1.474 eV), and d) 841.5 nm (1.473 eV). A spectrum from an emitting center is shown in every case, next to the corresponding map. The blue squares indicate the small areas where the spectrum in each map is taken.

In order to obtain at the same time information about the relative amount of iodide and lead, as well as structural features, in a large area of the two samples, we also performed scanning combined micro X-Ray Fluorescence/micro X-Ray Diraction (µXRF/µXRD) experiments on the XRD1 synchrotron beamline at ELETTRA, conrming the high sample homogeneity within the scanned area and a statistically signicant dierences of the I/Pb relative amount between the two samples (see Supporting Information for the details).

Optical characterization:

The structural dierences between the stochiometric and non-

stochiometric samples reect in their optical properties. Photo-luminescence (PL) spectra

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realized on the two samples at the cryogenic temperature of 4 K show that, while in the stoichiometric case the emission is a broad gaussian (Fig. 1d), in the non-stochiometric 2:1 FAPI perovskite, complex emission spectra with narrow peaks appear (Fig. 1h). To better investigate the optical properties of the non-stoichiometric sample, temperature dependent micro-photoluminescence measurements (see Methods) have been performed in the range 4 to 50 K on FAPI 2:1. In Fig. 2 the temperature dependent PL spectra of the sample measured with an excitation spot size of 1

µm

is shown. At 4 K we observe that

the emission is characterized by a wide peak, consistent with an emission from the bulk material, and very narrow peaks at lower energy, which disappear by increasing the temperature above 20 K. At higher temperatures, the narrow peaks vanish in favour of a new wide peak.

We exclude that this observation could be due to a phase transition, which,

usually, takes place at higher temperatures, in these materials.

34,35

We rather assign the ap-

pearance of these peaks at low temperature to emission from localized states. By spectrally resolving the spatial emission map when exciting the sample with an enlarged laser spot (20

µm

of spatial FWHM), a high density of localized bright spots towards the higher energies

of the spectrum is evidenced (Fig. 3a).

On the contrary, at lower energies (Fig. 3b-d), a

fewer, spectrally and spatially isolated emitters can be clearly distinguished, evidencing a clear strong spatial carrier localization in the non stochiometric FAPI. Analysis of the measurements performed on the samples obtained by changing the annealing temperature and stoichiometry evidences the eects of these process parameters in the optical properties. We found that while high annealing temperatures suppress the formation of conned domains, the unbalanced stoichiometry aects their spatial density (see Supplementary Material). To gather fundamental insights into the nature of these narrow emission peaks, PL experiments at dierent excitation powers were performed for a localized bright spot whose spectrum is shown in the inset of Fig. 4. The pumping power dependence demonstrates PL saturation of the emission intensity of these narrow peaks. As a comparison, a linear dependence from the bulk emission is also shown. This suggests an exciton quantum connement in the spatially

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localized emitting centers. In fact, as it is well known, the emission from zero-dimensional localized excitons reaches a saturation regime with increasing excitation power due to a maximum population value imposed by the Pauli exclusion. with the functional form

A(1 − e−Jσ ),

eective absorption cross section

σ.

where

J

A tting of the saturation curve

is the laser uence, gives the value of the

We obtained a value

σ ∼ 10−12 cm2 .

By comparing this

value with the absorption cross section of colloidal quantum dots whose diameters are well characterized in literature,

36

we obtain an indicative value of the size of these emitting cen-

tres of tens of nanometers, which is in good agreement with the HRTEM analyses.

Figure 4: Power dependence of the emission intensity from a localized emitter.

Second

The con-

tinuous lines show the tting of the emission dependence with power, using functions of −αP the form A(1 − e ) for the peaks, and linear for the bulk emission. As expected for two level emitters, there is a saturation with increasing power [peaks 1 (green circles) and 2 (red squares)], while the bulk PL has a linear dependence (blue diamonds). The inset shows a spectrum at low power, with both peaks and bulk emission indicated. The analysis is done on each peak separately by post-processing the spectra acquired at each power.

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Order Correlation:

Page 10 of 21

To verify the quantum nature of these emitters, the statistical prop-

erties of their PL intensity have been evaluated by measuring the second order correlation (g

(2)

) function.

This has been performed by using a Hanbury-Brown-Twiss (HBT) setup

(see Methods) by spatially and spectrally ltering (∼ 3 nm) the emitted light of individual emission centres. In order to reduce the relaxation time and background emission, we excited the samples at

780

nm (pulse duration of

3

ps; repetition rate 80 MHz). Results are

displayed in Fig. 5. The left panel shows the spectrum of the localized emitter for which the measurement was performed (the arrow indicates the peak on which the sured after being spectrally ltered), while the right panel shows the

g (2)

g (2)

has been mea-

measured in a time

window of 120 ns. The result evidences an antibunching behavior with a value of of

0.14 ± 0.05.

g 2 (τ = 0)

The small dierence from zero is possibly due to the unavoidable background

caused by the thick bulk emission, which is always present in the spectra and cannot be completely removed even after using narrow spatial and spectral lters. The

g (2) (τ = 0), the

saturation of the emission with power, and the very narrow linewidth (FWHM down to 0.36 nm), clearly conrm that emission from these conned spots in the unbalanced perovskite is associated to single quantum states. Pumping at 780 nm, a radiative decay time of

0.36

ns was measured on single perovskite

quantum dot(see Supplementary Material for a more complete discussion). This very short lifetime suggests that these kind of QDs could work at a repetition rate of GHz, oering a throughput comparable to InGaAs based QDs.

QD stability:

Finally, in order to check the stability of these bulk QDs, we recorded PL

signal from a single emitter as a function of time. The results are shown in Fig. 6. As can be seen from the color map of the PL recorded for 45 s of continuous excitation (Fig. 6) and for longer times till 90 minutes (see Fig.S10 of the SI), spectral diusion is within the resolution of the spectrometer and no blinking was observed. Noteworthy no sign of degradation of the material was detected during the full integration times of the from 1 to 10 hours.

10


g (2)

measurements, that varied

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Figure 5: Left: PL of a localized emitting centre. The emission is centred at has a linewidth of

823.2

nm and

0.5 nm.

Right: second order correlation function of localized PL measured 2 using a HBT setup. The g (τ = 0) = 0.14 ± 0.05 (antibunching) univocally indicates that

the narrow PL comes from an optically-active zero dimensional connement.

Figure 6: Upper panel: Time dependent PL spectra for a typical QD excited at a power of 2 1µW/µm . The time integration for each PL data is 5 s.

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Conclusions In conclusion, we demonstrate quantum connement emission in spatially localized quantum dots obtained by modifying the stochiometry of the perovskite thin lm. In particular, we ascribe the formation of such QDs to the presence of FAI in excess.

These single photon

emitters, which have been realized by using a very easy process, seem more stable and much faster than QDs formed in colloidal perovskite.

Methods and Experimental Sample preparation Materials:

Lead (II) iodide PbI2 ultradry 99.999 % (metals basis)

was purchased from Alfa Aesar, Formamidinium iodide HC(NH2 )2 I (FAI), Polyethylenimine 80% ethoxylated solution (PEIE), 2-isopropanol (IPA) and Dimethyl sulfoxide anhydrous, 99.9 % (DMSO) were purchased from Sigma Aldrich. All the materials were used as received without any further purication.

Perovskite thin lm preparation: ovskite

22

The stoichiometric and non-stoichiometric per-

precursors solutions were prepared by mixing FAI with PbI2 at the dierent ex-

plored molar ratio (1:1, 2:1, 3:1, 4:1), dissolved in DMSO with a concentration of 10 wt%

◦ with respect to the solvent. The solutions were stirred at 80 C for 30 minutes. Glass/PEIE modied substrates were prepared by depositing PEIE solution (0.05 wt% in IPA) at 5000 rpm for 40 sec in air. Then, the substrates were placed in glovebox and the perovskite precursors solutions were deposited in N2 atmosphere. The solutions were spin coated at 12000

◦ ◦ rpm for 100 s and two dierent annealing temperature were explored, 100 C and 170 C, for 10 min.

X-Ray measurements:

The X-Ray uorescence was detected by a Amptek X-123 SDD

detector, with a 60 seconds collection time per step. Monochromatic radiation of 16 keV (λ

12


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= 0.774909 ), with a 28

µm

(diameter) beam, was used to illuminate the samples. Samples

were mounted on a goniometer head for scanning in the XY plane (plane parallel to the sample and perpendicular to the X-ray beam). For each sample, more than 30 points on the XY plane have been analysed by collecting the uorescence signal for 60 seconds. Fluorescence spectra were rescaled so that the area under Pb-L3M5 is equal to 1 with the aim to remove dierence related to the intensity instability of the beam. performed by using RootProf software.

37

Rescaling and PCA have been

Fluorescence data belonging to the same cluster

have been averaged together to improve signal/noise ratio and analysed by using PyMCA.

TEM analyses:

38

High-resolution transmission electron microscopy (HRTEM) imaging,

high-angle annular dark eld-scanning TEM (HAADF-STEM) imaging and EDS analyses were performed on a FEI Tecnai G2 F20 (Schottky emitter) TEM, operated at 200 kV. The microscope is equipped with a Bruker X-Flash 6T30 EDS silicon-drift detector (SDD). The samples were obtained by spin-coating on amorphous-carbon-coated Cu grids for TEM (spin-coating on C lm side). They were kept in nitrogen-lled glove-box till shortly before the experiment.

Optical Properties:

For cryogenic temperatures measurements a microPL setup was

used in a epi-uorescence conguration. The samples were kept at cryogenic temperature (4 K) to avoid phononic eects. The samples were pumped with pulsed lasers either at with pulse duration of a repetition rate of a

60X

80

100

fs, or at

780

3

ps. Both lasers have

MHz. The pump laser was focused on the sample's surface through

microscope objective with a high numerical aperture (NA=0.7), that produced a

Gaussian prole with a space FWHM of

µm

nm with pulse duration of

420 nm

1 µm

(except for Fig. 3 in which the spot has 20

of FWHM). The collected PL was sent into a monochromator coupled to a CCD camera system

or into avalanche photodiodes (APD) detectors for time correlated single photon counting

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(TCSPC) measurements. Second order correlation functions and decay time on single QDs were measured by using a time correlated single photon counting module in the standard HBT conguration. For the g

(2)

experiments, integration times between 1 to 10 hours have

been used. Time resolved PL measurements have been performed triggering the signal with the laser pulse sent to one of the two APD.

Acknowledgement The authors acknowledge the ERC project ElecOpteR grant number 780757 and the project TECNOMED - Tecnopolo di Nanotecnologia e Fotonica per la Medicina di Precisione, (Ministry of University and Scientic Research (MIUR)Decreto Direttoriale n.

3449 del

4/12/2017, CUP B83B17000010001). G.G. gratefully acknowledges the project PERSEOPERrovskite-based Solar cells: towards high Eciency and lOng-term stability (Bando PRIN 2015-Italian Ministry of University and Scientic Research (MIUR) Decreto Direttoriale 4 novembre 2015 n.

2488, project number 20155LECAJ). A. R. gratefully acknowledges

SIR project "Two-Dimensional Colloidal Metal Dichalcogenides based Energy-Conversion Photovoltaics" (2D ECO), Bando SIR (Scientic Independence of young Researchers) 2014 MIUR Decreto Direttoriale 23 gennaio 2014 no. 197 (project number RBSI14FYVD, CUP: B82I15000950008). The Sta of the XRD1 beamline (M. Polentarutti, L. Barba, G. Bais, G. Chita) is acknowledged for their valuable support in the collection of XRF/XRD data. The authors thank Paolo Cazzato for technical support.

Supporting Information Available The following les are available free of charge.

•

Supporting Information le containing the paragraphs: Optimization of growth process; TEM characterization; XRD and XRF Experiments; Time dependent PL ex-

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periments; Statistics of the second order correlation function; Radiative decay time.

◦ The following Figures are included: PL spectra of samples annealed at 170 C with ◦ stoichiometry 1:1 and 2:1 (Figure S1); PL spectra of samples annealed at 100 C with stoichiometry 1:1 and 2:1 (Figure S2); PL spectra and real space emission images of

◦ samples annealed at 100 C with stoichiometry 1.5:1, 2:1 and 3:1 (Figure S3); PL spec◦ tra of non stochiometric FAPI 2:1 samples annealed at 100 C and obtained with a spin speed of 7000(a) and 12000(b) rpm (Figure S4); HRTEM image and FFT of the lm FAPI 1:1 (Figure S5); Composition of the 1:1 and 2:1 FAPI samples evaluated by STEM-EDS analyses (Figure S6); XRD patterns of stoichiometric and non stoichiometric FAPI (Figure S7); X-ray uorescence data for FAPI 2:1 and 1:1 (Figure S8); Ratio between the areas of the L3M5 transition of iodide and lead for FAPI 2:1 and 1:1 (Figure S9); Time dependent PL spectra for long times for a typical QD (Figure S10); Spectra of two representative quantum dots on sample FAPI 2:1 (Figure S11); Second order correlation function and decay time of representative quantum dots pumped at 780 nm and 420 nm (Figure S12);

Author Contributions D.G.S.F., L.P., M.D.G. and F. T. performed the optical measurements. A.G., A.R. and S.C. prepared the material and the samples. L.D., D.B. and V.A. performed data analysis. B. D. B., D.A. and C.G. performed XRD/XRF measurements and analyses. R.B. performed TEM measurements and analysis. L.D.M., G.G. and D.S. supervised the work related with the optical measurements, discussed and interpreted the data. A.L. and C.E.C supervised the work related with the sample preparation.

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Data availability The data that support the ndings of this study are available from the corresponding authors upon reasonable request.

Competing nancial interests The authors declare no conict of interest.

References 1. Eisaman, M. D.; Fan, J.; Migdall, A.; Polyakov, S. V. Invited Review Article: SinglePhoton Sources and Detectors.

2. Cuevas, Á.;

Rev. Sci. Instrum. 2011, 82, 071101.

Carreño, J. C. L.;

Silva, B.;

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