Inhibition of a structural phase transition in one-dimensional organometal halide perovskite nanorods grown inside porous silicon nanotube templates

One-dimensional organo-metal halide perovskite ($\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$) nanorods whose diameter and length are dictated by the inner size of porous silicon nanotube templates have been grown, characterized, and compared to bulk perovskites in the form of microwires. We have observed a structural phase transition for bulk perovskites, where the crystal structure changes from tetragonal to orthorhombic at about 160 K, as opposed to small diameter one-dimensional perovskite nanorods, of the order of 30–70 nm in diameter, where the phase transition is inhibited and the dominant phase remains tetragonal. Two major experimental techniques, infrared absorption spectroscopy and photoluminescence, were utilized to probe the temperature dependence of the perovskite phases over the 4–300 K temperature range. Yet, different characteristics of the phase transition were measured by the two spectroscopic methods and explained by the presence of small, tetragonal inclusions embedded in the orthorhombic phase. The inhibition of the phase transition is attributed to the large surface area of these one-dimensional perovskite nanorods, which gives rise to a large stress that, in turn, prevents the formation of the orthorhombic phase. The absence of phase transition enables the measurement of the tetragonal bandgap energy down to low temperatures.

Figure 1

Schematic view of the perovskite nanorods fabrication stages. Each scheme is composed of a top view of a single object and a side view of several objects on top of the substrate. (a) Growth of ZnO nanowires as a sacrificial template. (b) Chemical vapor deposition (CVD) of Si on top of the ZnO nanowires. (c) Template removal by etching with $\mathrm{N}{\mathrm{H}}_3/\mathrm{HCl}$ gas mixture to create empty nanotubes. (d) PSiNTs immersed in in 1:1 solution of $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{I}$ and $\mathrm{Pb}{\mathrm{I}}_2$. (e) Spin coating followed by thermal annealing to create PSiNTs loaded with perovskites. Steps (d) and (e) are repeated three times to ensure uniform loading of the perovskites. (f) Perovskite loaded PSiNTs.

Figure 2

The TEM images showing (a) empty PSiNT and (b) perovskite loaded PSiNT.

Figure 3

The 3D plots of the temperature-dependent PL spectra versus photon energy for (a) bulk perovskites (microwires) and (b)–(d) 175 nm, 70 nm, and 30 nm perovskite nanorods, respectively.

Figure 4

Comparison of the normalized PL spectra of the bulk perovskites (black line) and the 1D perovskite nanorods (blue, green, and red lines are related to nanorods of 175, 70, and 30 nm in diameter, respectively) at different temperatures across the phase-transition temperature (of about 100–150 K).

Figure 5

The relative area of the high-energy PL line (associated with the orthorhombic phase), normalized to the total area of both the low-energy (associated with the tetragonal phase) and the high-energy PL lines, for the bulk (black sybmols and line), 175 nm nanorods (blue), 70 nm nanorods (green), and 30 nm nanorods (red). All data are related to results of the temperature-dependent PL spectroscopy shown in Figs. 3 and 4.

Figure 6

Temperature dependence of the PL peak energies associated with the tetragonal phase (low-energy PL peaks; red symbols and lines) and the orthorhombic phase (high-energy PL peaks; blue symbols and lines), for (a) bulk perovskites, (b) 175 nm nanorods, (c) 70 nm nanorods, and (d) 30 nm nanorods. Notice that the orthorhombic peaks energies (blue symbols and lines) can be resolved for bulk and 175 nm perovskite nanorods and at temperatures below 140 K only. The solid lines represent the best fit of the experimental data to Eq. (1) with the fitting parameters summarized in Table 1.

Figure 7

The IR absorption spectra of (a) bulk perovskites (microwires), (b) 175 nm nanorods, and (c) 30 nm nanorods at various temperatures from 4 K up to 180 K. The black vertical dotted lines represent the two vibrational modes (at $919\mathrm{c}{\mathrm{m}}^{-1}$ and $1468\mathrm{c}{\mathrm{m}}^{-1}$) chosen to characterize the orthorhombic and the tetragonal phases respectively.

Figure 8

The relative area of the orthorhombic phase (the $919\mathrm{c}{\mathrm{m}}^{-1}$ rocking mode), normalized to the total area of the orthorhombic plus the tetragonal ($1468\mathrm{c}{\mathrm{m}}^{-1}$ bending mode) phases, for bulk perovskites (black symbols and line), 175 nanorods (blue symbols and line), and 30 nm nanorods (red symbols and line). All data are related to results of the temperature-dependent IR absorption spectroscopy shown in Fig. 7.

Figure 9

Schematic bandgap-energy drawings of the orthorhombic phase ($E_g^O)$ and the small tetragonal inclusions having smaller bandgap energy ($E_g^T<E_g^O$). As a result, charge carriers that are excited at the orthorhombic phase via photon absorption are trapped in the tetragonal inclusions and radiatively recombine from that phase. The values of the bandgap energies of the two phases can be found in Table 1.