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ZnO as a wide band-gap semiconductor material is a candidate material for many applications in device technology, e.g. as transparent window in solar cells, in ultra-violet (UV) lasers and light emitting diodes, and as field emitters in flat-panel displays [1]. Since emission of electrons at lower voltage occurs easier from sharp tips the control and optimization of tip morphologies in aligned nanorod arrangements is of particular interest [1].Metal-organic chemical vapour deposition (MOCVD) without catalyst has been used to grow ZnO nanorods with tips on Si (111) and Si (001) substrate surfaces. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and cross-section transmission electron microscopy (TEM) were combined for characterising morphology and structure of the aligned ZnO nanorods grown under varying MOCVD conditions.For the synthesis of the ZnO nanorods high-purity diethyl zinc (DEZn) and N 2 O were used as Zn and O sources, respectively, and triethylgallium (TEGa) for Ga doping. The growth procedures consisted typically of the following steps: (i) growth of a nucleation layer (350°C-400°C for 3-30min) followed by an annealing step (600°C-650°C) (ii) synthesis of nanorods (600°C-650°C for3 0min) at flow rate ratios Zn:N 2 O:Ga of typically 30:30:2 (sccm) to 50:50:2 (sccm) and higher (iii) tip growth on rods by further increase of flow rate ratios for time intervals between 1-2min. Fig.1 shows examples of aligned arrangements of ZnO nanorods with different tip morphologies. The XRD pattern analyses (Fig.1 right) indicate that the ZnO exhibits hexagonal wurtzite structure. The ZnO [001] axis directions are preferentially aligned parallel to the respective Si <111> and to the Si <001> substrate normal directions. This result is also confirmed by selected-area electron diffraction (SAD) experiments (Fig.3 inset). SAD experiments performed on individual nanorods show their single-crystalline structure (Fig.2 inset). The SEM images (Fig.1 left) and the TEM images of tips ( Fig.2 left center) reveal that the tip regions possess surface protrusions, partly with facets, of ≤ 5 nm in height. Fig.3 shows examples of bright-field (BF) and dark-field (DF) crosssection TEM images (compare for SEM image Fig.1, left) of the spindle-like crystalline ZnO nanorods with dimensions of~1500nm in length and up to~200nm in width (Fig.3). The nanorods emerge from a poly-crystalline layer about 200 nm thick on a thin amorphous interface layer (presumably SiO 2 ). The room-temperature photoluminescence (PL) spectrum (Fig. 2 right) exhibits strong ultraviolet emission at~384 nm which can be attributed to direct recombination of free excitons [2]. The negligible intensity between 500 and 650 nm attributed to ionized oxygen vacancies [2] indicates a high structural quality of the ZnO nanorods.These results indicate that under the applied conditions nanorod growth occurs preferentially along [001] ZnO directions resulting in aligned arrangements of single-crystalline ZnO nanorods of fairly high structural quality. Nanorod morph...