**2. In situ techniques**

Observing and characterizing the nanowires while they are growing is called in situ growth monitoring. Strictly speaking, 'in operando' is the exact word, but we stick to 'in situ' to conform to popular usage. In situ techniques can provide directly interpretable and time-resolved observations enabling better understanding of the growth mechanism, which in turn empowers better control of nanowire growth for specific technological applications.

In situ characterization of nanowire crystal structure or nanowire morphology has been reported using various techniques. In situ RHEED attached to MBE systems can be used to follow crystal structure changes and nucleation/birth of ensemble of nanowires [50, 51]. By modifying the optical reflectometry techniques that have been used conventionally in MOCVD systems, the nanowire diameter and length evolution has been monitored in situ in real time for an ordered array of nanowires [52]. Combining finite difference frequency domain simulations with in situ reflectometry enabled monitoring growth of randomly positioned nanowires (i.e. periodic array was not a necessity) [53]. In situ X-ray diffraction (XRD) has been used to study crystal phase of the nanowire [54, 55] and the catalyst phase [56]. In situ infrared spectroscopy has been used to correlate surface chemistry during nanowire growth to its morphology [57–59] or the choice of growth direction [60]. Line-of-sight quadrupole mass spectrometry in situ was used to study different stages of nanowire growth including nanowire nucleation [61]. All these techniques give ensemble averaged results.

In situ imaging techniques on the other hand allows monitoring individual nanowires. Optical microscopes due to the limited spatial resolution are not ideal for observing growth evolution of nanowire (though some studies have been attempted using confocal optical microscopy using photoluminescence measurements [62]). Scanning electron microscopes (SEMs) have better spatial resolution than optical microscopes and could be used to monitor nanowire growth [63–67]. In situ SEM combined with Auger electron spectroscopy has been used to correlate nanowire growth and morphology to surface chemistry [63]. In situ electron backscattered electron diffraction (EBSD) performed during growth in an SEM has been used to study crystal phases and crystallographic orientation [64]. An SEM uses electron scattering from a sample while a transmission electron microscope (TEM) uses the electrons transmitted through a thin sample (preferably less than ~50 nm) to form images. TEMs have better spatial resolution than SEMs. Be it in an in situ SEM or TEM study, a video or a series of images are captured to study the dynamics of the process in relation with the specimen environment. A key advantage of using in situ microscopic techniques, particularly in situ TEM, is that localized or dynamic behavior happening at individual wires could be investigated. One limitation to studying nanowire growth inside a microscope is that electron microscopes require vacuum environment to minimize electron scattering in the air outside the specimen. So, often the growth conditions, e.g. pressure, used for the in situ growth study are slightly modified compared to a conventional growth method. Typical total pressures used in conventional ex situ CVD are much beyond the maximum attainable pressure for in situ TEM experiments. Majority of the pressure in the ex situ CVD case is from the carrier gas. By careful design of the TEM and the growth chamber, it is in principle possible to obtain comparable precursor partial pressures.
