Nonthermal Plasma-Synthesized Phosphorus–Boron co-Doped Si

However, we find that larger diameter PB:Si NCs could not be prepared in this ... The solid black horizontal line illustrates the position of the bulk...
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Article Cite This: Chem. Mater. 2019, 31, 4426−4435

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Nonthermal Plasma-Synthesized Phosphorus−Boron co-Doped Si Nanocrystals: A New Approach to Nontoxic NIR-Emitters Rens Limpens,*,† Gregory F. Pach,† and Nathan R. Neale* National Renewable Energy Laboratory, Golden, Colorado 80401, United States

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S Supporting Information *

ABSTRACT: We report on the successful creation of nonthermal plasmasynthesized phorphorus and boron co-doped Si nanocrystals (PB:Si NCs) with diameters (DNC) ranging from 2.9 to 7.3 nm. Peak photoluminescence (PL) emission energies for all PB:Si NC diameters are ca. 400−500 meV lower than the excitonic emission values in intrinsic Si NCs, which can be attributed to prevalent donor−acceptor (D−A) transitions within the co-doped system. This D−A transition model is further evidenced by PL lifetimes within the range of 40−80 μs, faster than what is observed for intrinsic Si NCs. By reducing the level of confinement within PB:Si NCs (i.e., DNC > 4 nm), we are able to red-shift the nearinfrared (NIR)-emitting D−A transitions to below the band gap of bulk Si (1.12 eV). We quantify the PL quantum yield (PLQY) for a range of DNC and show that the plasma method can achieve reasonably high PLQY values (12% for DNC = 2.9 nm), even without any optimization of the synthesis or surface chemistry. We posit that perfect charge compensation cannot explain these results and propose a model in which dominant n-type doping accounts for the observations. Ultimately, these results demonstrate that nonthermal plasma synthesis is a viable pathway for preparing PB:Si NCs featuring NIR sub-band gap D−A transitions with relatively high quantum yields. More generally, this study provides insight into how doping affects energy and charge transfer within quantum-confined systems.

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INTRODUCTION The optoelectronic, photovoltaic, biomedical, and communications application potential of colloidal semiconductor nanocrystals (NCs) has attracted great research interest in recent decades. The optical character of NCs can be controlled simply by tuning the degree of quantum confinement (size and shape). Different semiconductor materials in combination with changes in the NC surface termination can be used to optimize the emission/absorption spectra, radiative lifetimes, and the conductivity of solid-state NC arrays for the intended application. Compound semiconductor NCs based on elements from the II−VI and IV−VI families have demonstrated commercial viability in optoelectronic devices and biomedical applications.1−6 Scalability, high-yield synthesis, high optical efficiencies, emission tunability, and low toxicity/ biocompatibility are the crucial pillars on which commercial viability rests. However, the vast majority of semiconductor materials active in the infrared are dominated by those containing toxic elements (Pb, As, Te, Hg, Se).7 Group IV NCs based on Si and Ge are often proposed as less toxic alternatives,8−10 but the optical quality and emission tunability of these materials generally have lagged behind their compound semiconductor cousins. In the last decade, significant improvements have been reached in terms of the high-yield synthesis of optically efficient Si NCs,11−13 with work of Mangolini et al.14,15 in particular catalyzing renewed interest in group IV NCs in which nonthermal plasma synthesis was used to produce dodecyl-capped Si NCs © 2019 American Chemical Society

exhibiting 60% photoluminescent quantum yield (PLQY). Plasma-grown Si NCs have been leveraged into solutionprocessed electroluminescence16 and Schottky junction solar cell devices17 and provide a platform for further tuning their optical properties via controlled surface chemistry.18 Another exciting development with Si NCs is that the emission tunability window, typically limited to ∼1 μm owing to the Si bulk band gap of 1.12 eV, recently has been extended to longer wavelengths. This was accomplished by the use of donor−acceptor (D−A) transitions, giving rise to size-tunable near-infrared (NIR) radiative recombination while maintaining the desirable microsecond radiative lifetime typical of the indirect band gap transition in Si NCs.19−23 The creation of these D−A transitions is established by simultaneously codoping the Si NCs with phosphorus (P) and boron (B) impurity atoms. Charge compensation in PB-pair complexes allows for optically active D−A transitions without the interference of charge-induced nonradiative Auger recombination (AR).24 Hence, these systems combine the advantages of Si NCs (size-tunability, low toxicity/biocompatibility, and relatively long radiative recombination rates) with the effect of co-dopants that provide long wavelength NIR emission features approaching the short-wave infrared (1.4−3 μm).25 An initial study on carrier multiplication in these exotic Received: February 26, 2019 Revised: May 7, 2019 Published: May 9, 2019 4426

DOI: 10.1021/acs.chemmater.9b00810 Chem. Mater. 2019, 31, 4426−4435

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nanostructures shows that these sub-band gap transitions can facilitate the carrier multiplication process at a lower threshold energy than through intrinsic band edge transitions.26 For a theoretical review regarding doped Si NCs, we refer the interested reader to ref 27. Until now, the production of the PB co-doped Si (PB:Si) nanostructures has been limited to the temperature-induced growth of borophosphosilicate matrices, leading to solid-state dispersions of SiO2-embedded PB:Si NCs.19,20 In a second step, liberation of the NCs is performed by hydrofluoric acid etching to allow for NC dispersion in polar solvents such as methanol and water.28−30 PLQY values of 1.55 eV (with PLQYs reaching >10%); however, we believe that this increase in PLQY at high energies most likely reflects the transition to excitonic band edge emission. The relatively low PLQY of D−A recombination and its size dependence is surprising since the emission lifetime (20−80 μs) is roughly the same regardless of NC size. The authors attribute this lack of correlation between size, lifetime, and PLQY to the existence of a large number of dark NCs32 resulting from the imperfect charge compensation, particularly for the smaller NCs.31 Nonthermal plasma techniques have been used to dope Si NCs with phosphorus and boron individually,33,34 and our prior reports demonstrate the ability to effectively incorporate these dopants into the Si NC core across a broad range of NC diameters.35,36 However, the production of a PB co-doped Si NC system via nonthermal plasma synthesis, to the best of our knowledge, has remained unreported. To this end, nonthermal plasma synthesis provides a unique pathway to efficiently produce doped Si structures as compared to more conventional, thermodynamically driven processes. These more conventional methods rely on leaving the targeted system at an elevated temperature (typically >900 °C) for an extended period of time (>1 h) to allow dopant atoms to diffuse into the Si lattice. During nonthermal plasma growth, NCs reside in the plasma for millisecond time scales that are insufficient for dopant atoms to diffuse in, or out, of the NC core. Instead, dopant atoms become irreversibly “trapped” within the NC during the kinetic growth process as small atomic and molecular intermediates nucleate into clusters and small NCs, which then undergo agglomeration and subsequent growth within the active plasma region. In this work, we leverage high purity, nonthermal plasma synthesis to create hydrogen-terminated, free-standing PB:Si NCs in a proof-of-concept approach, analogous to the work of Mangolini for intrinsic Si NCs.15 First, we demonstrate that it is possible to synthesize PB:Si NCs with optically active NIR D−A transitions through a plasma-induced kinetic method. Second, we compare these PB:Si NCs to those obtained through thermodynamic growth. Third, we comprehensively characterize the photophysics of these plasma-synthesized PB:Si NCs through standard spectroscopies (Fourier transform infrared (FTIR), PL, time-resolved PL (TRPL) and PLQY) in combination with ultrafast kinetics of the photoexcited e−h+ pairs by pump−probe induced absorption (IA) spectroscopy. With the obtained detailed insights, we provide experimental context and discuss the promise of nonthermal plasma synthesis of these exotic NIR-emitting nanomaterials.

Article

EXPERIMENTAL SECTION

General. All PB:Si NCs were produced by a nonthermal plasma synthesis method using a capacitively coupled 13.56 MHz RF plasma. The NCs were synthesized using a 25 mm OD/19 mm ID quartz reactor tube. NC sizes were controlled by varying the pressure in the reactor tube (between 1 and 3 Torr) as well as Ar/H2 carrier gas ratios. Table S1 summarizes the synthetic methods for all PB:Si NCs reported. Hydrogen-terminated PB:Si NCs (PB:Si−H) were collected downstream from the plasma on a 400-mesh stainless steel filter and transferred via load-lock to an inert-atmosphere, argon-filled glovebox for collection. CAUTION! Air oxidation of PB:Si NCs generates toxic phosphine gas! Surface Functionalization. PB:Si−H NCs were functionalized via a hydrosilylation reaction with alkenes using rigorously purified reagents and air-free conditions, following previous work.18,37 Briefly, NC powders were suspended directly in 1-octadecene and heated at 220 °C for ∼2 h. Functionalized NC solutions were then diluted in toluene and washed via precipitation using acetonitrile as the antisolvent as in our prior work.18,37 PL. Steady-state PL measurements were taken using a home-built system. Samples were excited using a Thorlabs fiber-coupled 405 nm light-emitting diode (LED) pulsed at 10 Hz using a Thorlabs DC2200 LED driver. Visible detection was made using an Ocean Optics OceanFX spectrometer while NIR detection was made using an Ocean Optics NIRQuest spectrometer. Spectra were stitched using a LabVIEW program developed in-house. Detector calibration was done using an Ocean Optics HL-2000-HP blackbody lamp. PLQY. Absolute PLQY measurements were conducted using a fiber-coupled sphere (Labsphere 3P-GPS). A 532 nm collimated laser diode (Thorlabs CPS532-C2) was used as the excitation source. The sphere was fiber-coupled, using a Thorlabs M35L02 fiber (Ø 1000 μm), to a Princeton HRS-300-SS spectrograph including a PYL100F Si CCD. For further details, we refer the interested reader to Figure S8. Relative PLQY measurements were conducted on the steady-state PL system, as described above. As-prepared NC powders (PB:Si−H) were deposited on stainless steel meshes, and PL intensity was collected through a Thorlabs reflection probe. TRPL. Photoexcitation was conducted by using a Nd:YAG-pumped optical parametric oscillator (Spectra Physics Quanta Ray and GWU Premi Scan). Photoluminescence was detected in off-axis front-face geometry with an InGaAs avalanche photodiode (APD) (Thorlabs APD430C) attached to the other side, transducing transient optical signals with DC-400 MHz bandwidth. Combination of a 650 and 700 long pass filter was used to block the excitation light (405 nm), and the signal from the APD was digitized using a fast oscilloscope (Tektronix DPO7254). FTIR. FTIR absorbance measurements were conducted in the inertatmosphere glove box on a Bruker Alpha FTIR spectrometer using a diffuse reflectance infrared Fourier transform spectrometer attachment with a resolution of 4 cm−1. Reflective gold-coated polished Si wafers were used for background measurements, and PB:Si−H NCs were deposited by mechanically pressing powder directly onto the same substrates. Spectra were baseline-corrected using the concave rubberband correction method (typically ≤4 iterations). X-ray Diffraction (XRD). XRD spectra were taken on a Bruker D8 Discover diffractometer using Cu Kα radiation (λ =1.54 Å). Toluene slurries of the NCs were deposited on Si-based zero diffractions plates. To calculate the NC sizes from the XRD patterns, we used the

(

Scherrer-approach DNC =

kλ w cos(c)

), with D

NC

the diameter of the

NC, as shaping factor (k) we used 1.1, λ is the X-ray wavelength, and w and c the width and center of the Si(111) diffraction peak. Induced Absorption. Ultrafast time-resolved measurements were collected by making use of a pump−probe technique with a femtosecond transient absorption spectrometer (Helios, Ultrafast Systems). As a laser source, we used a 4 W Ti-sapphire amplifier (Libra, Coherent), operating at 1 kHz and 100 fs pulse width. The fundamental beam (800 nm) was split, and a small portion ( 3.35 nm show a peak emission

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RESULTS PB:Si NCs are produced via nonthermal plasma synthesis using a capacitively coupled 13.56 MHz nonthermal plasma of process gases (SiH4, PH3, and B2H6) with additional carrier gases (Ar and H2) in a quartz tube (Figure 1, Scheme 1). Nonthermal plasma synthesis of Si NCs is known to create surfaces well-passivated with hydrogen atoms,14 resulting in *SiHx surface groups (where *Si denotes a surface Si atom and x = 1, 2, or 3) and characterized by stretching frequencies from 2080 to 2150 cm−1 in the infrared region.37 Figure 1a shows the FTIR spectrum of a typical hydrogen-terminated PB codoped Si NC system (PB:Si−H), and FTIR spectra of all NC diameters (DNC) are presented in Figure S1. In addition to stretching vibrations from *SiHx, νP−H surface vibrational modes are distinctly present at 2276 cm−1 in addition to typical *SiHx deformation modes (864−906 cm−1). XRD patterns indicate the crystalline nature of the NC systems (Figure 1b; see Figure S2 for all XRD patterns). NC sizing is approximated by applying the Scherrer broadening analysis to the (111) peak (see Experimental Section for details), and these values are confirmed by a size analysis of transmission electron microscopy (TEM) images, as shown in Figure S3. Highresolution TEM (HRTEM) imaging indicates that the crystalline lattice extends all of the way to the surface (Figure 1c), showing that the co-doped NC samples are single crystalline. Energy dispersive X-ray spectroscopy (EDS) on a 3.95 nm diameter PB:Si−H NC sample was used to confirm the presence of both phosphorus and boron within the codoped sample (Figure S4). Although surface P atom sites are clearly present as evidenced by the *PHx stretching vibrations, no distinct 4428

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Figure 2. PL spectroscopy of plasma-synthesized co-doped PB:Si−H NCs. Excitation is performed at a wavelength of λexc = 405 nm. (a) PL emission spectra as a function of the NC size (DNC = 2.9−7.3 nm). The band gap of bulk Si 1.12 eV (1107 nm) is depicted by the solid black vertical line. All dashed lines are log-normal fits. (b) Peak PL emission energy as a function of NC diameter (red circles). Tight-binding calculations of donor−acceptor pair recombination levels in PB co-doped Si NCs, for 5 (blue open triangles) and 10 (blue open squares) PB pairs.25 The black and red dashed lines indicate the peak PL emission size dependence for intrinsic37 and PB co-doped Si NCs, respectively. The solid black horizontal line illustrates the position of the bulk Si band gap (1.12 eV). The blue dashed lines are power law fits and function as a guide. Scheme 3 shows the NIR emission mechanism from P atom donors to B atom acceptors.

partially unscreened Coulomb interactions resulting in defect levels that lie much deeper than what is typically observed for impurity states in bulk Si.25 Interestingly, these calculations reveal that close (but not neighboring) PB pairs in the core are responsible for these optically active D−A transitions, and that the D−A transition energy can be tuned by the number of PB pairs. To estimate the number of PB pairs responsible for the observed PL transitions, we include two sets of Delerue’s calculations on D−A recombinations,25 representing PLpeak positions of PB:Si−H with 5 (blue open triangles, Figure 2b) and 10 (blue open squares) active PB pairs. These calculations describe a situation of 40 random configurations of the impurity atom positions, and the displayed values represent the mean value at each NC diameter. The comparison of these curves with our data indicates 5−10 active PB dopant pairs per PB:Si NC. Based on this analysis, we conclude that the nonthermal plasma method is quite efficient at PB incorporation in the NC core at substitutional sites for the whole NC size range explored here. This likely results from the kinetically driven growth mechanism that traps dopant atoms in, rather than expelling them from, the NC core. We next explore the photophysical details of these heavily doped nanoparticles and leverage analytical methods recently developed by us in a series of papers on singly doped P:Si35,36 and B:Si35 NCs and glass matrix-derived PB co-doped Si NCs.24,44 In these works, we showed that excess charges can induce an efficient trion recombination in the nanosecond time regime and thus dominate the carrier dynamics relative to the much slower (∼100 μs)32 radiative recombination pathway. Here, we perform similar experiments on optically transparent, colloidal solutions that are formed by functionalizing the surface of PB:Si NCs with alkyl groups via radical-initiated reactions between *SiHx and 1-octadecene, termed PB:Si−R (Figure 3, Scheme 4; see Methods and Figure S7 for details). Figure 3a displays the absolute PLQY of PB:Si−R NCs as a function of the PL peak emission energy (see Figure S8 for all PLQY measurements). The magnitude of the PLQY directly

wavelength above the band gap of bulk Si (1107 nm), as depicted by the solid black line. We quantify the emission wavelength dependence by plotting the PLpeak as a function of the DNC (red dots, Figure 2b). The size dependence is well fit with a power function (red dashed line) E(D NC) = 0.84 + 4.35D NC−2.09

(1)

yielding the optical transition E (in eV). Typically, the Si NC band gap size dependence is predicted following the effective mass approximation37 or the linear combination of atomic orbitals41 models describing DNC−2 and DNC−1.39 behavior, respectively. In a prior work, we showed that the band gap behavior of intrinsic Si NCs from a dozen experimental reports is not fit by either of these models, and thus we developed a sizing curve with DNC−1.69 (the black dashed line in Figure 2b)37 that closely follows a third theoretical model based on atomistic pseudopotential calculations.42 A direct comparison of this intrinsic PLpeak-size dependence with the PB:Si NCs shows a consistent 400−500 meV red-shifted emission for the co-doped NCs, in line with what is expected for the donor− acceptor (D−A) pair recombination.23,43 The full width at half-maximum (FWHM) values of these plasma-produced codoped particles are similar to oxide matrix-grown PB:Si NCs (Figure S6). Importantly, no band edge emission is observed over the range of sizes explored in this work, which shows the potential of the nonthermal plasma method for producing these exotic co-doped nanostructures. Furthermore, the absence of band edge emission indicates that carrier relaxation from band states to D−A levels is faster than the microsecond excitonic band-to-band radiative recombination rate. Delerue25 exploited atomistic tight-binding calculations (using the spatial arrangement of point charges in a dielectric sphere) to define the Coulomb potential, i.e., the specific energy levels, which both confirm the experimental data and give insight into the broadening of the emission spectra and the microscopic electronic mechanisms. The relatively deep defect levels (compared to typical defect levels in bulk Si) are perfectly explained by a dielectric confinement effect, causing 4429

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Figure 3. (a) PLQY values of PB:Si−H and PB:Si−R as a function of the peak PL emission (eV). The solid red circles represent absolute PLQY values taken from alkyl-functionalized PB:Si−R NCs dispersed in TCE, performed at an excitation wavelength of λexc = 405 nm. Relative PLQY values, as measured from the as-prepared H-terminated PB:NC−H powders, are shown by the red open circles, these values have been scaled to coincide with the absolute values. The red dashed curve is a power law fit as a guide to the eye. The black “+”-symbols indicate absolute PLQY values of thermodynamically grown PB:Si from Sugimoto et al.31 (b) Time-resolved PL transients for typical PB:Si−R NCs (in red, DNC = 3.95 nm) and typical plasma-produced intrinsic Si−R NCs (in gray, DNC = 4.2 nm). Dashed lines represent single-exponential fitting curves. (c, d) Effective PL lifetime, taken from the single-exponential fits, as a function of the peak emission wavelength (panel c) or NC size (panel d) for the PB:Si−R (red solid circles), in comparison to the excitonic band gap emission from intrinsic Si NCs32 (black dashed lines). Artistic Scheme: Credit Alfred Hicks, NREL.

These values compare well with the 20−80 μs lifetimes reported by Fujii et al. for DNC = 3.5−5.8 nm glass matrixgrown PB:Si NCs31 suspended in methanol as well as the sizedependent 10−100 μs radiative lifetimes calculated from the density functional theory for a D−A transition in a Si NC with a single PB pair.47 The difference between the excitonic band edge emission lifetimes from intrinsic Si NCs and D−A emission lifetimes from PB:Si NCs becomes smaller with decreasing NC size, and the extrapolation of the data suggests that the effective lifetimes of excitonic and D−A transitions coincide for sub-2.9 nm NCs. These results are in line with a carrier localization effect of the impurity state in addition to the well-known confinement effect of the nanoparticle.43 In this model, the D−A acceptor states are still treated with the same k-space arguments that govern the pseudo-direct band gap Si nanoparticle48,49 but with a more relaxed k-vector due to an enhanced spatial confinement induced by the impurity level, which facilitates the radiative transition. In the strong quantum confinement regime (i.e., for small NCs with DNC < 3 nm), the additional carrier localization effect of the impurity state is negligible compared to the confinement potential of the NC, and the D−A acceptor transitions can be treated following Heisenberg’s uncertainty principle identical to the intrinsic excitonic states, leading to D−A lifetimes that are similar to the exitonic transitions. We next performed ultrafast induced absorption (IA) spectroscopy measurements to provide additional insights into the intriguing photophysics of these PB:Si NCs. As mentioned, our recent ultrafast studies on doped Si NCs24,35,36,44 lay the framework of these investigations in which we have shown that the presence of a single trion

correlates to the emission energy, with the highest reported PLQY of 12% at a PLpeak of 1.51 eV. The PLQY sharply decreases with smaller PLpeak and drops below 1% for emission energies 60% PLQY (at around DNC ∼ 3.5 nm) has only been observed for intrinsic Si NCs produced from the nonthermal plasma synthesis with a secondary H2 injection step to optimize the surface structure.15 No such optimization has been attempted here, and future research exploring the effects of such a secondary H2-gas injection is therefore highly interesting. Time-resolved PL measurements (TRPL) demonstrate that these PB:Si−R NC samples exhibit μs decay that is slightly size-dependent (Figure 3b−d; the complete set of TRPL traces are presented in the Supporting Information Figure S9). A dominant, near single-exponential decay component is observed for all PB:Si−R samples such that the effective lifetime of this channel is retrieved by a single-exponential fit (dashed red line, Figure 3b). As shown in the summary plots in Figure 3c,d, these D−A transitions exhibit PL lifetimes between 40 and 80 μs (red solid circles), which is slightly faster than what is typically observed for intrinsic Si−R NC systems (black dashed lines) over a similar NC size range.32 4430

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Chemistry of Materials (consisting of a neutral exciton and a single free carrier) provides a nanosecond recombination channel due to Coulomb-driven Auger recombination. This holds true for both the negative (resulting from a free electron) as well as the positive (free hole) trions.35 Considering the dominating nature of trion recombination (ns) over the relatively slow radiative transition (μs),50 the high PLQYs for PLpeak > 1.1 eV in Figure 3a, therefore, only can be explained by an absence of free carriers (in which we define a free carrier as an ionized donor or acceptor that is delocalized with respect to the individual NC). The IA measurements in Figure 4 confirm this deduction for DNC < 5 nm. In this pump−probe experiment, the photoexcited e−h+ pairs are created using a pump wavelength λ pump = 600 nm (Scheme 5, Figure 4). Subsequently, the time-dependent concentration of excited states is monitored through the intraband absorption of the free carriers, which is typical for indirect band gap materials,24,51 using a mechanical delay stage including a probe wavelength of λprobe = 1200 nm. No dependence on probe wavelength was observed within the wavelength range of the experimental setup (900−1500 nm, Figure S10), showing that this technique is valid even for a range of individual NC sizes within the same ensemble. Figure 4a displays the ultrafast carrier dynamics within the first 2.5 ns of creation within PB:Si−R NCs in comparison to a typical intrinsic Si−R NC trace. The traces of the PB:Si−R with DNC < 5.0 nm (here represented by DNC = 2.9 nm, blue open circles in Figure 4a) all show similar kinetics and are nearly identical to those of intrinsic Si NCs (DNC = 4.2 nm; see Figure S11 for all kinetics). The main difference in dynamics between small ( Dcritical. To confirm that these observations are in line with the AR model of trion recombination, we measured the biexciton lifetime (τXX) of our PB:Si−R NCs. Conveniently, τXX is sensitive to the doping type and concentration and can be used as a powerful tool in characterizing doped NC systems.44 In essence, it is expected that the lifetime of a charged biexciton state is faster than that of a neutral biexciton state, simply due to the presence of an additional initial state (i.e., a five-body interaction between a biexciton and an e− or h+ compared with a four-body interaction in a neutral biexciton). This is exactly what is observed experimentally for such charged NCs, and we refer the interested reader to our previous detailed investigation of AR in singly P- and B-doped Si NCs.35 In

Figure 4. IA spectroscopy on PB:Si−R NCs with pump and probe wavelengths of λpump = 600 nm and λprobe = 1200 nm. (a) Singleexciton carrier dynamics (Δt < 2500 ps) of PB:Si−R with DNC = 2.9 and 5.0 nm (in blue and red, respectively). In black, a typical intrinsic Si NC trace (DNC = 4.2 nm). (b) Quantification of nonradiative quench in the carrier dynamics by plotting the intensity ratio of the kinetics of 1 ps and 2 ns (I1ps/I2ns) as a function of the NC size. The shadow area indicates DNC > Dcritical. (c) Biexciton lifetime as a function of the NC volume. The red dashed line is a linear fit. The shadow area indicates VNC > Vcritical.

the present work, we find that for DNC < Dcritical (equivalently, for VNC ≤ 52 nm3), τXX of PB:Si−R NCs follows a linear dependence on the NC volume (Figure 4c, red dashed line) as is universally observed for both direct and indirect semiconductor nanoparticles.52,53 Critically, these τXX values for PB co-doped NCs are identical to those for intrinsic Si NCs over the same size range24,35,44,50,51 (see Figure S13 for the experimental details of their derivation), which conclusively demonstrates the absence of free carrier interactions with the 4431

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Chemistry of Materials photoexcited e−h+ pairs in the small VNC ≤ 52 nm3 PB:Si NCs. Indeed, as expected from our previous discussion, for VNC > 52 nm3, τXX values are shortened relative to the linear trend in intrinsic Si NCs, thereby proving the existence of electronic interactions between the photoexcited e−h+ pairs (the two excitons that form the biexciton in this case) and dopinginduced free carriers. This aligns with the critical size model for free electron creation in plasma-produced Si NCs35 and explains the fairly low PLQY values below PLpeak of ∼1.1 eV that is only achieved for NCs larger than Dcritical.

substitutional P sites relative to B sites is present in PB:Si NCs prepared via the nonthermal plasma method. This conclusion aligns well with the reported higher doping efficiency for P vs B-atoms.60 After all of the holes are compensated by the abundance of electrons, the remaining uncompensated electrons are localized at DNC < Dcritical (correlating to a PL peak emission energy above 1.1 eV for the PB:Si−R NCs in this work, Figure S7). This hypothesis explains the optical activity of the D−A pairs in that small size regime, without the need for perfect charge compensation. Scheme 6 schematically

DISCUSSION All of the evidence (FTIR, PLQY, and IA) show that these heavily co-doped NCs do not contain free carriers for DNC < Dcritical. In the discussion below, we consider whether the observed absence of free carrier effects on the vibrational (Figure 1a) and emission (Figure 2) spectra as well as ultrafast dynamics (Figure 4) is equivalent to “perfect” charge compensation. For perfect charge compensation to occur, an exactly equal number of active substitutional P and B dopants must be present in a NC. Although it is theoretically possible to match the average dopant concentrations, doping processes are governed by Poisson statistics,54 inherently leading to individual NCs containing a distribution of dopant concentrations. Following this logic, it is statistically impossible to reach perfect charge compensation in PB co-doped Si NC ensembles that would be free from nonradiative Auger interactions. As noted previously, the nonthermal plasma synthesis method relies on kinetic rather than thermodynamic growth.55,56 As a result, particles are created within the plasma on the order of milliseconds,14,57 which avoids the purging of single dopants out of the NC core58 via the so-called “selfpurification effect” commonly observed in thermodynamically driven processes.59 These mechanistic differences might suggest that plasma-derived samples contain more charge imbalance (and therefore exhibit lower PLQYs) than for those prepared from an oxide matrix. We find that the opposite effect on the PLQY is observed, with the small PB:Si NCs in this work exhibiting superior PLQY values at emission energies >1.4 eV compared with those from PB:Si NCs in a glass matrix grown under thermodynamic control. To account for this unexpected result, we propose an alternative explanation in the form of an electron-donor activation energy argument. Our recent work on P:Si embedded in a glass matrix44 indicates the presence of a critical NC size (Dcritical ∼ 6 nm) above which doping-induced carriers are free carriers. For DNC < Dcritical, a combination of the quantum confinement effect and unscreened Coulomb interactions increases the activation energy that is required to create free carriers to such an extent that the energy bath at room temperature is insufficient to “free” the doping-induced carriers. Therefore, the carriers remain localized to their original doping atom and do not contribute to the carrier dynamics.24 We conducted a similar investigation on both P:Si−R (electrons) and B:Si−R (holes) plasma-produced Si NCs35 and, importantly for the conclusions in the present work, showed that (1) free electrons in P:Si−R NCs are not created at DNC less than ∼4 nm and (2) no critical size is observed for the holes in B:Si−R NCs over a size range of DNC = 3−7 nm. This Dcritical ∼ 4 nm threshold for creating free electrons in P:Si−R NCs is exactly the same critical NC size as found here for plasma-synthesized PB:Si−R NCs. Using this framework, we propose that an abundance of

Scheme 6. Charge Compensation and Photophysics of PB:Si under Dominant n-Type Conditions

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illustrates the proposed scenario where charge compensation allows the optical D−A transition to occur, and the localization of remaining doping-induced electrons does not impact the radiative recombination via AR losses at DNC < Dcritical. The PB:Si NC ensembles with DNC ≥ 5.0 nm (i.e., DNC > Dcritical) confirm our reasoning of the incomplete electron compensation by indicating the existence of a small amount of free electrons. This is observed indirectly through their low PLQYs ( Dcritical, but, similar to the free-carrier insensitivity of the TRPL technique, their presence is not detected through FTIR spectroscopy. This seems counterintuitive since free carriers typically can be detected through their collective electromagnetic oscilations (i.e., localized surface plasmon resonance, LSPR, modes)61 that lead to relatively broad FTIR absorption spectra centered around 1000−3000 cm−1 in the case of Si NCs.33,35,36 The absence of these LSPR modes in the FTIR spectra for DNC > Dcritical indicates that the number of free electrons is relatively low ( 4.6 nm (Figure 4c) are longer by a factor of 3 compared with heavily n-type-doped P:Si−R NCs (≤100 ps).35 As such, the FTIR and IA data allow us to quantify that a small, single-digit number of free electrons is present in these PB:Si NCs. The corroboration of the FTIR 4432

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Chemistry of Materials and IA data thus shows that only a relatively small amount of free electrons are present (not affecting the FTIR spectra and moderately altering the photophysics), which are sensitively probed through the IA spectroscopy. On a last note we mention that the NC diameter is not the only parameter dictating the level of quantum confinement, and the critical NC size of ∼4.5 nm is, therefore, not an absolute metric and is likely influenced by the matrix environment, surface chemistry, and related factors. For example, in a recent publication, we were able to compress the exciton wavefuntion in Si NCs by changing the head group of saturated surface ligands,18 which should induce slight changes in the critical NC size between as-prepared and functionalized PB:Si. As such, we argue that the PLpeak (for PB:Si−R, a DNC of ∼5 nm relates to a peak PL emission energy of ∼1.1 eV, Figure S7) directly relates to the overall level of quantum confinement, and the PLpeak is, therefore, a useful indicator of the free carrier regime. This nuance of the NC environment could explain slight differences in the critical size (ranging from DNC ∼ 4−6 nm) between differently prepared and functionalized Si NCs in our previous works.35,44

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (R. L.). *Email: [email protected] (N. N). ORCID

Rens Limpens: 0000-0002-2417-9389 Gregory F. Pach: 0000-0002-5062-735X Nathan R. Neale: 0000-0001-5654-1664 Author Contributions †

R.L. and G.F.P. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors are employees of the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Project concept, data collection, and analysis were funded under the National Renewable Energy Laboratory (NREL) LDRD program, Nozik Director’s Postdoctoral Fellowship. Synthesis, surface functionalization, structural characterization, and data analysis were funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program. We thank Andrew Norman for his assistance with the HRTEM and EDS experiments.

CONCLUSIONS These results show that the nonthermal plasma method is promising as a source of high-quality, low-toxicity NIRemitting PB co-doped Si NCs. By using a kinetic growth approach, we show that the efficient D−A recombination is not limited to structures that rely on low formation energies of PB pairs during the thermodynamic growth. In the present stage and without optimization via an additional hydrogen surface passivation, we report on PLQYs of up to 12% for such nonthermal plasma-grown PB:Si NCs. Since the ultrafast kinetics indicate that the optical quality of these PB:Si NCs might surpass that of their intrinsic cousins, it is possible that the optical quality could potentially exceed the 60% PLQY benchmark value for optimized intrinsic Si NCs. To explain the high PLQYs in these heavily doped nanostructures, we propose a model in which (1) the creation of 5−10 PB pairs in each NC allows for the optical D−A transition to occur and (2) the localization of remaining doping-induced electrons at DNC < Dcritical (with Dcritical ∼ 4.5 nm) suppresses the interference of nonradiative Auger recombination and facilitates radiative D− A recombination. Since holes do not localize at sizes down to at least 3.0 nm, high optical quality PB:Si NCs only result from a net n-type doped system. With the increased fundamental understanding of these P and B co-doped Si NCs, we hope to inspire future research to investigate the optical limits of these promising, low-toxicity NIR-emitting nanoparticles, to ultimately realize their optoelectronic, photovoltaic, biomedical, and communications potential. More generally, this study provides insight into how doping affects energy and charge transfer within nanocrystals relevant to upconversion, downconversion, sensitization, and related systems.

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formation in NMP, and FWHM of PL emission peak of PB:Si−H; EDS spectrum, FTIR spectrum, XRD powder patterns, PL spectra, absolute PLQY measurement data, TRPL data, IA data, and derivation of biexciton lifetimes for PB:Si−R (PDF)

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b00810. Experimental parameters for the plasma synthesis of all PB:Si NCs; FTIR spectra, XRD powder patters, size histogram, optical image of the spontaneous solution 4433

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