Figures 1(a)–1(d) show the high-magnified FESEM micrographs of CdS:Ni(II)X NBs by samples A-D, representing the NiF₂-, NiCl₂-, NiBr₂-, and NiI₂-doped CdS samples by CVD growth in the tube furnace. The morphology of doped samples seemed to be affected by the initial precursors [34] but no significant changes. Au nanoparticles were often seen as a catalyst on the top of all the belts, confirming their early vapor–liquid–solid (VLS) growth mechanism and then some VS growth to the belt. A similar growth mechanism and optical luminescence in NiCl₂-doped CdS nanoribbons have been studied in a previous report [32].

All the samples (A, B, C, and D) were investigated with FESEM-EDX which confirmed that NBs are composed of dominant elements Cd, S, and Ni; sample A-D EDX spectra are displayed in Figure S1(a–d), in which there is no F and Cl elements found in the samples A and B, while the Br and I elements have been detected in samples C and D because of their larger atomic number in the gas-phase growth. Furthermore, their elemental distribution was probed by EDX mapping (Figures 2(a)–2(d)) for the Cd, S, and Ni elements. Cd, S, and Si are dominant; minor Ni (854 eV), I (618 eV and 386 eV), and O (530 eV) elements can also be detected. The Si element comes from the substrate; O should come from the surface-adsorbed O₂ and O element in the surface of silicon wafer.

The crystallinity of the doped NBs was analyzed by high-resolution TEM (HRTEM). Figure S2(a–d) shows the HRTEM images of sample A to sample D, respectively, in which there are atomic separation fluctuations of 0.358, 0.357, and 0.359 nm, respectively, which is the atomic spacing value change of (100) plane for the hexagonal CdS structure. The slight variation of the “d” value might be due to different atomic doping of Ni²⁺ and X^- into the CdS lattice. In the HRTEM image, we can also see the varied contrast distribution and sparsely cloudy atom order, which is different from the clean order for the pure crystal. That cloudy stuff usually comes from the dopants in CdS lattice. Insets are the corresponding selected area electron diffraction (SAED) patterns confirming that the NBs are a single crystalline CdS wurtzite structure.

X-ray diffraction (XRD) patterns of four differently CdS:Ni NBs are shown in Figure S3(a–d). In all the cases, the prominent peaks at 24.2, 26.4, 28.2, and 47.7° are originated from the (100), (002), (101), and (110) planes of the wurtzite CdS structure (JCPDS card no. 65-3414), and the XRD profiles of (100), (002), and (101) peaks are the same for the NiCl₂-, NiBr₂-, and NiI₂-doped CdS NBs with the strongest (101) peak, while the NiF₂-doped CdS NBs have a different profile with its strongest (002) peak. No peaks related to any other lattice phases of CdS and NiX₂ were found, matching with their EDX results. For comparison, the X-ray diffraction pattern of pure CdS NBs is presented in Figure S3(e), whose profile is much like that of Ni(Cl, Br, I)₂-doped NBs. All the diffraction patterns were fitted with the multipeak Gaussian function (insets of Figure S3(a–d)) and obtained FWHM along with precise peak positions. All the peak shifts and FWHM are listed in Table S1, in which there is a small shift for the doped belts, as compared to pure NBs. This shift may be due to the lattice strain caused by the incorporation of doping atoms.

CdS crystallize in a hexagonal wurtzite structure which belongs to the P6₃mc-C⁴_6v space group, and it has nine optical phonon modes along with three acoustic phonon modes [35]. Figure S4(a–d) shows the Raman spectra of four different samples and pure CdS NB (Figure S4(e) Raman spectra of pure CdS NB) measured in the spectral range 180-750 cm^-1 at room temperature by 532 nm excitation. In CdS crystal, the macroscopic electric field associated with longitudinal optical (LO) vibrations make the LO mode energy greater than the transverse optical (TO) mode energy. This effect removes triple degeneration in the Brillouin zone center, producing double degeneration of LO mode and single degeneration of TO mode. The identity of these modes is well established by their polarization characteristics as described in Ref. [35]. On the basis of the above-mentioned group theory explanation, the observed five modes are attributed to 1TO, E₁(TO), E₂^H, 1LO, and 2LO phonon modes of CdS [36, 37]. All the modes are listed in Table S2; most of the modes redshifted as compared to respective modes of pure CdS NB, confirming the successful incorporation of Ni ions or halogen ions into CdS lattice [38].

The magnetic properties are investigated by measuring M-H curves in the range -20000 to 20000 Oe using VSM. Figures 3(a)–3(d) represent the magnetic response of samples A, B, C, and D, respectively, at room temperature. Magnetic hysteresis loop of Si substrate and pure CdS NBs demonstrate diamagnetic properties (not shown here). After doping with NiX (X: Cl₂, F₂, Br₂, and I₂), the NBs demonstrate ferromagnetism. The M‐H curves show good magnetic hysteresis loops, clearly indicating the ferromagnetic behaviour. Their saturation magnetization M_{\text{s}}, remanence M_{\text{r}}, and coercivity H_{\text{c}} can be roughly obtained in Table S3 for the perpendicular detection. It can be seen from Table S3; sample D show the strongest magnetic signals among the four samples while the weakest magnetic signals occur in the NiF₂-doped CdS NBs. The anisotropic magnetic effect [39] and Ni-Ni coupling [32] in NiX cluster should be the cause of its magnetism. These results indicate that these CdS NBs doped by NiX₂ are diluted magnetic semiconductors.

In a dilute magnetic semiconductor, minor doping often produces inhomogeneous doping ion distribution; both single TM ion and coupled TM ions contribute to the magnetic responses in wide-band II-VI DMS [40]. So, the accurate determination for the magnetic parameters is hard to realize.

For the photoluminescence (PL) study, Figures 4(a)–4(d) show the PL spectra of all the samples (A, B, C, and D) under the 405 nm CW laser. All the PL spectra are recorded in the range 450 to 1000 nm. Figure S5 shows the PL of pure CdS NB, whose narrow emission band centered at about 508-510 nm is attributed to the bandedge (BE) emission from the free exciton recombination of CdS crystal. There is no broadband detected in the longer wavelength range, confirming free of Cd or S vacancies under such growth condition [41, 42]. Figures 4(a)–4(c) show PL spectra for the individual NBs of sample A, sample B, and sample C, respectively. Samples A and B show only two emission bands, one lying at near bandedge (NBE) and the other lies at the near infrared (NIR) region. The former comes from FX+EMP, while the latter should come from d-d transition of Ni ion and (NiX){}_n (X = S or F and Cl, n\ge 2) [32, 33, 43]. In contrast, sample C shows three emission bands, two bands lying at NBE region and one at NIR region. In all three A, B, and C samples, their first peaks all slightly redshifted as compared to that of the pure CdS NB as shown in Table S4. This slight redshift indicates that the first emission band should contain the contribution of exciton magnetic polaron (EMP) [7, 30]. The emission band in the range from NIR of 700–850 nm in all three samples is attributed to d-d transitions of Ni ion or (NiX){}_n aggregates [32], whose d levels have minor dependence on the weak crystal field effect of Ni ion [44], but the covalence effect takes effect in the order of NiI₂ > NiBr₂ > NiCl₂ > NiF₂. In samples A and B, the dominant d-d transition bands are located at 768 nm and 756 nm for CdS:NiF and CdS:NiCl, respectively [30]. In sample C, the bands similar to the CdS:NiF NB have been detected; moreover, one new emission band appeared at 561.4 nm. This emission band at 561.4 nm is partially attributed to the high d-d level transition between ³A₂g and ¹E_g of Ni ions in bromides [45] or the self-trapped exciton due to Br ions in CdS lattice [46]. The latter assignment should be more favourable here, which will be discussed later.

Figure 4(d) shows the PL spectrum of individual NBs of sample D which is obtained from the chemical vapor deposition of CdS with NiI₂ in the gas phase. Although these belts contain dopant ions as confirmed by EDX (Figure S1), XRD (Figure S3), and Raman analysis (Figure S4), they show completely different emission spectra from those of samples A, B, and C. First, there is a very large redshift and even broader emission band centered at 543.4 nm than those for other Ni compound-doped NBs, far from the bandedge emission at 506 nm for pure CdS. Second, there is no prominent emission band detected in NIR region as that in Figures 4(a)–4(c). Extending and comparing with the emission profile of CdS:NiBr NB, we did careful fitting on the broad emission band of CdS:NiI and obtained the separated bands at 473 nm, 530 nm, 572 nm, 644 nm, and 773 nm. The first one at 473 nm may be the partial existence of bound magnetic polaron (BMP) pair due to the free exciton coupling with antiferromagnetic-coupled Ni ions in CdS lattice [47] that can only happen at more p-d hybridization and covalence in iodides [44]. The 530 nm band should be the EMP, that is, FX coupled with ferromagnetically coupled Ni(II) clusters [7] in CdS lattice, which should be the most stable excitation near the bandedge of CdS:NiI NB for the low doping level. The 573 nm band should be the self-trapped exciton [46] due to I^- ion in CdS lattice. Its assignment is reasonable here that the self-trapped exciton component by independent I^- ion should have more contribution and a lower energy than Br^- in NiBr₂ doping system because I^- could be more easily reduced and easily deposited than Br^- in CdS lattice. The broadband at about 644 nm should contain the d-d transitions of Ni ions [32, 33, 43] and their aggregates due to its very broad width. We also found a 773 nm band but has a very low ratio, which should be due to the magnetic polaron pair. Our assignments were also supported by the EDX result as shown in Figure S1(d); the Ni concentration (at 854 eV) is higher than I (at 618 eV), though adsorbed O 1s were also detected. In Figure S1(c), the EDX spectrum of CdS:NiBr contains Ni and Br elements, but the spectra of CdS:NiCl and CdS:NiF contain no Cl and F elements, respectively. These results prove that the existence of Br and I in the lattice has relations to the self-trapped exciton in CdS:NiX NBs. Hence, the above assignment of the very broad emission band around 543 nm in CdS:NiI₂ NB, which is far from the bandedge (508 nm) of CdS crystal and the d-d transition (650-700 nm) of Ni ions, should contain some mobile and wide-band, covering the range of 450-800 nm due to Ni-I charge-transfer. This band can combine with the conduction band (above 500 nm) of CdS, EMP, self-trapped exciton, and the d-d transition of Ni ions. Such a combined state may participate in the emission of CdS:NiI, but not all the above individual states contribute to radiative transition as in CdS:NiF, CdS:NiCl, and CdS:NiBr. Figure S6 present all the reflection spectra of different compound-doped CdS NBs; only the NiI₂-doped NBs show significant absorption in the range of 510-650 nm, which is in agreement with the above-combined state, especially the Ni-I charge-transfer band. The occurrence of the combination state may reflect another electronic profile with those diluted magnetic semiconductor without doping negative ions like I or Br, i.e., the charge-transfer band between dopant ions like Ni-I may involve the combination via p-d hybridization, which bring up the formation of magnetic polarons and polaron-polaron interaction [47]. Therefore, 473 nm and 530 nm bands are the antiparallel and parallel-coupled spin states out of Ni ions coupling to the free exciton in CdS lattice, respectively.

From the doping situation, the band structures of NiI₂-doped CdS structure could be calculated. Except self-trapped exciton, other states could be seen in their band structure as shown in Figure S7 by VASP ab initio calculation. If I element is codoped within the CdS lattice, the charge-transfer bandedge of NiI cluster could merge with the up d state above the Fermi level, which may take part in the optical d-d transition. If I ion is not doped into lattice, this cannot happen. In fact, we found that both Br and I could be easily left into the CdS lattice from their micro-photoluminescence spectra in Figure 4. As the phenomena described above, we can figure out the Ni and X doping spatial diagram (a) in DMS CdS lattice and the band or level profile (b) of different states as shown in Figures 5(a) and 5(b). The single Ni, I, coupled Ni, and coupled Ni+I doping states coexisted in the lattice (Figure 5(a)), forming an inhomogeneous DMS structure. They can form different levels within the bandgap out of their different interactions in CdS lattice (Figure 5(b)). These levels may lead to novel optical processes, especially the one coupled to the d state.

To know this unusual emission behaviour of sample D as compared to samples A, B, and C, we excited their single NBs with a femtosecond laser pulse of 405 nm. Surprisingly, samples A, B, and C did show the lasing line near CdS bandedge only at very low dopant concentration as usual [7], but no other lasing lines as shown in Figure S8. The lasing profiles of CdS:NiF₂, CdS:NiCl₂, and CdS:NiBr₂ nanobelts in Figure S8 show similar lasing profile with varied threshold powers. For CdS:NiF₂ NBs, sometimes, it may show double lines near the bandedge (514-525 nm) at initial excitation due to overdoping (\text{Ni}>5\%) and varied magnetic phases, but at higher power, one mode lasing line still dominated. We observed that only CdS:NiI₂ belts showed dual lasing lines with large span at high power in different transition zones out of its unique character.

High doped (over 1% Ni) CdS NB often could not produce the lasing line at all. However, sample D NBs showed a variation of both spectral and lasing profiles as shown in Figures 6(a)–6(d). For low excitation fluence (10 kW/cm²), NB showed the only broad emission band centered at 534.5 nm (EMP at 530 nm+self-trapped exciton at 572 nm) and weak band-tail from 630 to 800 nm (d-d transition). But when increasing the excitation fluence (≥70 kW/cm²), its spectra are shown in Figure 6(b), in which an extremely very narrow lasing line (width 1.5 nm) appears at 789.3 nm, but the green emission band at 535 nm did not give a narrow line though its intensity increases. Further increasing the pump fluence from 70 to 180 kW/cm², the narrow line at 789.3 nm rises, and another very narrow line at 530.9 nm appears, which is shown in Figure 6(c). Interestingly, with further increasing pump fluence up to 300 kW/cm², the line at 530.9 nm increases much faster than the line at 789.3 nm, which was shown in Figure 6(d). After the pump fluence over 350 kW/cm², the line at 789.3 nm increases faster than the other one as shown in Figure 6(e). When the pump fluence goes up to 400 kW/cm², both sharp lasing lines increase superlinearly further without saturation as shown in Figure 6(f). Figure 6(g) shows the excitation fluence-dependent intensities for lines at 530.9 nm and 789.3 nm, respectively. Lasing thresholds for lines 789.3 nm and 530.9 nm are seen to be 70 and 180 kW/cm², respectively, as shown in Figure 6(h). This fact proves that the Ni and I element incorporations in the lattice of CdS NB play an important role in their dual-color lasing profile.

Lasing characteristics are further confirmed by their PL lifetime measurements. Figures 7(a) and 7(b) show the decay curves for 530.9 and 789.3 nm emissions of CdS:NiI₂ NB at the pump fluences of 180, 300, 350, and 410 kW/cm², respectively. Their single-exponential decay function and biexponential decay function were used to fit the PL decay curves presented in Figures 7(a) and 7(b), respectively. Their lifetime variations with excitation powers were listed in Table S5. For 530.9 nm emission, the first-lifetime component decreases from 0.95 ns to 0.28 ns as the laser fluence increases from 100 to 410 kw/cm², whose ratio increases from 89% to 98% at initial, then decreases to 72%. The second lifetime component does the same thing from 3.72 ns to 1.12 ns. Their lifetime decreasing and intensity superlinear increasing indicate a stimulated emission process with the rising excitation power, in which the EMPs were produced at first and then they were converted into a coherent state—collective EMPs (CEMP) at high excitation [7, 48]. Over a critical power, the ratio of second lifetime component decreases with rising power reflecting the occurrence of some competition processes or scattering effect, which could be identified by the threshold value for lasing in Figure 6(g).

For the 789 nm emission, the first component lifetime increases at powers below its threshold value (70 kW/cm²), while the second component shows a tendency towards lifetime reduction as shown in Figure 7 and Table S5, combining with their superlinear intensity rising. The second component always has the dominant ratio. This emission process presents a clear coherent radiation process. Compared with the EMP behaviour in CdS:Mn [30] and CdS:Co NB [7], the emission bands at 530.9 nm and 789 nm in CdS:NiI₂ NB have clear lifetime profiles of the EMP and LEMP, respectively; both of them can convert into CEMP for coherent emission. Their lifetime variations hint that these EMPs have even larger stabilities and faster populations than other states, stronger emission coherence, which supply more probabilities for CEMP formation, and related coherent radiation transition. Two CEMP cooccur in the same location in DMS. What does that indicate?

All the chemicals were purchased from Alfa Aesar and are of analytical grade. A high-temperature furnace equipped with a quartz tube was utilized to grow the NBs. For the collection of nanomaterials, n-type Si (111) was used as substrates. For this purpose, firstly, Si substrates were sonicated in acetone, isopropanol, and deionized H₂O for 15 min., respectively, and then dried with pressurized N₂ flow. After cleaning, a thin layer of Au (~5 nm) was deposited on these substrates with the help of a Quorum (Q150S) sputter coater and loaded into the downstream region of the quartz tube. 1.5 g mixture of pure CdS (99.999%) and X (X: NiF₂, NiCl₂, NiBr₂, and NiI₂) nanoscale powders was used as a precursor. These precursors were loaded in a ceramic boat and inserted into the middle region of the quartz tube. To achieve the best crystallinity of the nanomaterials, oxygen-free environment was obtained inside the quartz tube with the help of a mechanical and turbo molecular pump. After that, the furnace temperature was raised to 750°C at the rate of 15°C/min and then stayed at 750°C for 25 min. During the reaction, the flow rate of Ar/10%H₂ carrier gas was maintained to 10 sccm. After 25 min of reaction, a thick yellow-colored product was collected on the Si substrates. The product obtained from CdS:NiF₂, CdS:NiCl₂, CdS:NiBr₂, and CdS:NiI₂ precursors was named as sample A, sample B, sample C, and sample D, respectively.

Yellow-colored product morphologies were investigated by field emission scanning electron microscopy (FESEM, FEI Quanta 450 FEG) attached with energy-dispersive X-ray spectroscopy (EDX) Thermo (Noran System 7: Ultradry 30 mm²) facility. The transmission electron microscopy (TEM) observations were carried out on a Hitachi H-800 microscope with an accelerating voltage of 200 kV. Phase purity of the as-prepared products was investigated by X-ray diffraction (XRD, Bruker: D8-Advance with Cu Kα radiation of wavelength 1.5418 Å). The 2θ ranges from 20 to 60° with a step of 0.01° with a count time of 2 s. Raman experiments were conducted in a Raman scattering spectrometer (LABRAM-HR, JY, Horiba), with the 532 nm Ar laser as the excitation source. Photoluminescence (PL) spectra were obtained by Laser Confocal PL microscopy (Horiba JY-iHR550, Olympus BX51M) using 405 nm continuous wave (CW) semiconductor laser as an excitation source. The laser beam was focused onto the sample by a microscope with a 50x objective lens. PL lasing spectra were measured by using Ti-sapphire laser (coherent, 405 nm, 130 fs, 80 MHz) as a pumping source. The fs-laser beam size was optimized using the lens in front of the microscope to give a circular beam spot diameter of 3 μm to ensure uniform illumination of the doped NBs, and the output signals were collected and analyzed by a spectrometer. PL lifetime decay profiles were recorded by the time-correlated single photon counting (TCSPC) system (PicoQuant “TimeHarp 200”). The pulsed diode laser at 405 nm (tens of ps pulse-width) was used (PicoQuant “PDL 800-B,” 10 MHz-80 MHz) as the excitation source. The measurements of magnetization (M) as a function of H were characterized in a VSM in the range -2000\le H\le 2000\text{Oe}.